The Vertebral Column

The vertebral column (or spine) extends from the skull to the tip of the tail. It consists of a large number of separate bones, the vertebrae, firmly but not rigidly joined together. It serves to stiffen the body axis and thus contributes to the maintenance of posture; by alternate flexion and extension, and sometimes by torsion, it plays a part in progression and other activities. The vertebral column encloses and protects the spinal cord and accessory structures within a central canal; in a more general way, it shields the structures of the neck, thorax, abdomen, and pelvis (see Figure 2–1).

Most vertebrae conform to a common pattern on which are superimposed features that distinguish the several regions: cervical (neck), thoracic (back, in the narrow sense), lumbar (loins), sacral (croup), and caudal (tail). The numbers of vertebrae that compose these regions vary among species and also, although to a much smaller extent, individually. They can be represented by a formula: that for the dog is C7, T13, L7, S3, Cd20–23.

A typical vertebra (Figure 2–6) consists of a massive body surmounted by an arch that completes the enclosure of a vertebral foramen; it is the summation of these foramina that constitutes the vertebral canal. The body, broadly cylindrical, is somewhat flattened on its dorsal surface, which faces into the vertebral canal; it may carry a median crest ventrally. Its extremities are usually curved: the cranial one is convex, the caudal one concave. The arch consists of two upright pedicles, and from each of these a lamina projects medially to meet its fellow and thus complete the ring about the spinal cord. The bases of the pedicles are notched, and when successive bones articulate, these notches combine to outline intervertebral foramina, openings through which pass both the spinal nerves and the vessels that supply the structures within the vertebral canal. Sometimes an additional lateral vertebral foramen perforates the pedicle next to the intervertebral foramen.

Figure 2–6 Lumbar vertebra of the dog, left lateral view. 1, Spinous process; 2, cranial articular process; 3, transverse process; 4, body; 5, caudal vertebral notch; 6, arch; 7, caudal articular process.

Each vertebra also carries a number of processes. The dorsal or spinous process springs from the union of the laminae and is generally prominent, although its form, its length, and its inclination vary with the region and with the species. Transverse processes project to each side at the junction of the body and the arch; these processes arise at the level of the intervertebral foramina and divide the muscles of the trunk into dorsal and ventral divisions. Synovial joints connect restricted parts of the arches. Sometimes the articular facets hardly rise above the level of their surroundings, but elsewhere, and especially in the caudal thoracic and lumbar region, the facets are carried on articular processes that project cranially and caudally from the dorsal portions of the arches (Figure 2–6/2,7).

In domestic as in almost all mammals there are seven cervical vertebrae. The first two, the atlas and the axis, are much modified to allow free movement of the head and require individual description. The remaining five are more typical.

The atlas is the most unusual of all the vertebrae because it appears to possess no body but to consist of two lateral masses joined by dorsal and ventral arches (Figure 2–7, A). This form results from the fusion (in early embryonic life) of a component of the atlantal body with the corresponding part of the following bone, the axis. This addition provides the axis with a cranial projection (dens; Figure 2–7, B/5), which fits into the vertebral foramen of the atlas and serves as a pivot around which the atlas (and the head) may be rotated. A plate of bone, the wing of the atlas (ala atlantis, transverse process), projects laterally from each mass, constituting a landmark that is often visible and always palpable in the living animal. The cranial aspect of the ventral arch and the adjacent areas on the wings carry two deep excavations that receive the occipital condyles of the skull. These facets approach ventrally, and in some species they merge. The caudal aspect of the ventral arch is hollowed transversely to provide an articular surface that engages with the cranial extremity of the axis. An extension (fovea dentis; Figure 2–7, A/2) of this facet onto the dorsal surface of the ventral arch accommodates the dens. The dorsal arch is perforated by openings that correspond with the transverse and intervertebral foramina of more typical cervical vertebrae; in some species a third (alar) foramen perforates the wing.

Figure 2–7 Cervical vertebrae of the dog; cranial is to the left. A, Atlas, dorsal view. B, Axis, lateral view. C, Fifth vertebra, lateral view. 1, Wing of atlas; 2, fovea dentis; 3, lateral vertebral foramen; 4, transverse foramen; 5, dens; 6, spinous process; 7, caudal articular process; 8, transverse process; 9, body; 10, cranial articular process; 11, position of vertebral foramen.

The axis is the longest vertebra. Its cranial extremity carries the dens, which is rodlike in carnivores and more spoutlike in some other species. The cranial extremity of the body and the ventral surface of the dens concur in forming a single wide articular surface for the atlas. Dorsally the dens is roughened for the attachment of ligaments that hold it in place. The arch carries a very high (and in the dog, long) spinous process that bears articular facets at its caudal extremity; these meet corresponding facets on the third cervical vertebra. The transverse processes are large; each is perforated toward its root by a transverse foramen that transmits the vertebral artery, vein, and nerve.

The remaining cervical vertebrae become progressively shorter as the series is followed toward its junction with the thorax. The extremities of the body are more strongly curved than in other regions and slope obliquely. The ventral surface carries a stout crest. The arch is strong and wide, but the spinous process is poorly developed except on the last (considerable variation, however, exists among species). The large transverse process (Figure 2–7/8) branches into dorsal and ventral tubercles, the latter commonly developing a caudal platelike extension (Figure 2–8/5). On the third to sixth bones the process is perforated by a transverse foramen through which the vertebral vessels and nerve pass. The articular facets are large and flat but do not rise above the surrounding level. The seventh cervical vertebra, transitional to those of the thoracic region, is distinguished by its taller spinous process, unperforated transverse process, and the presence of facets on the caudal extremity of its body for articulation with the first pair of ribs.

Figure 2–8 Nuchal ligament of the dog. 1, Wing of atlas; 2, spinous process of axis; 3, nuchal ligament; 4, spinous process of first thoracic vertebra; 5, platelike extension of transverse process.

The thoracic vertebrae (Figure 2–9) articulate with the ribs and correspond with these in number. Minor variations in number are not uncommon; they are often compensated by a reciprocal change in the lumbar region that leaves the thoracolumbar total unaffected. All thoracic vertebrae share common features, but serial changes also occur that gradually (and on some points abruptly) distinguish the more cranial from the more caudal bones. Common thoracic features are short bodies with flattened extremities; costal facets, on both extremities for the rib heads and on the transverse processes for the rib tubercles; short, stubby transverse processes; closely fitting arches; very prominent spinous processes; and low articular processes.

Figure 2–9 Thoracic vertebra of the dog; left lateral view. 1, Spinous process; 2, caudal articular process; 3, transverse process with costal fovea; 4, mamillary process; 5, caudal vertebral notch; 6, 7, costal foveae; 8, body.

Conspicuous serial features are a rapid increase in the height of the spinous processes, which reach a maximum a few vertebrae behind the cervicothoracic junction and gradually decline thereafter; progressive simplification of the costal facets (those on the transverse processes approach and finally merge with those on the cranial extremity); reduction (and eventual disappearance) of the caudal costal facets; and appearance of an additional (mamillary) process as a projection from the transverse process and its gradual migration to join the cranial articular process. More abrupt changes toward the end of the thoracic series include sudden alteration from a caudodorsal to a craniodorsal orientation of the spinous processes and a change in the character of the articular facets from the cervical to the lumbar pattern (Figure 2–10). In some species, including the dog, the last members of the thoracic series possess yet other (accessory) processes that spring from the caudal part of the arch to overlap the following bone.

Figure 2–10 Contrast the orientation (arrows) of the articular surfaces of a cervical (left) and a lumbar (right) vertebra of the dog, caudal view.

The lumbar vertebrae (Figure 2–11) differ from the thoracic vertebrae in the greater length and more uniform shape of their bodies. Other regional features are absence of costal facets; a shorter height and generally forward slope of the spinous processes; long, flattened transverse processes that project laterally, sometimes (as in the dog) with a cranioventral inclination; interlocking articular processes; and prominent mamillary, and sometimes also accessory, processes.

Figure 2–11 Lumbar vertebrae of the dog, left lateral view. 1, Mamillary process; 2, accessory process; 3, spinous process; 4, transverse process; 5, body; 6, intervertebral disk.

Caudal to the loins the vertebral column is continued by the sacrum, a single bone formed by the fusion of several vertebrae. The sacrum forms a firm articulation with the pelvic girdle through which the thrust of the hindlimbs is transmitted to the trunk. Usually only one or two of the constituent vertebrae directly participate in the articulation. The more caudal bones project behind this to furnish the greater part of the roof of the pelvic cavity. In some animals (especially pigs) one or more tail vertebrae may be incorporated into the sacrum in later life. In the dog the three sacral vertebrae form a short quadrilateral block (Figure 2–12).

Figure 2–12 Canine sacrum and caudal vertebrae. A, Sacrum, ventral view. B, Sacrum, dorsal view. C, Sacrum, cranial view. D, Caudal vertebra, dorsal view. E, Caudal vertebra, cranial view. 1, Promontory; 2, auricular articular surface; 3, ventral (3′ dorsal) sacral foramina for ventral (3′ dorsal) branches of sacral nerves; 4, spinous process; 5, rudimentary articular process; 6, vertebral canal; 7, body; 8, transverse process; 9, hemal arch, also called chevron; 10, cranial articular process.

The sacrum commonly narrows from its cranial to its caudal extremity and is curved along its length to present a smooth, slightly concave face toward the pelvic cavity. In most species the dorsal surface is marked by the appropriate number of spinous processes, although these may be much reduced or even absent (e.g., pig). When present, they may preserve their independence (e.g., dog or horse) or fuse to form a continuous crest (e.g., ruminants). Lateral to this, a lower irregular crest usually marks the site of the redundant articular processes. The margin of the bone is formed by the fused transverse processes and carries toward its cranial extremity the articular surface for the ilium; this is often “ear-shaped,” hence the name auricular surface (Figure 2–12/2).

The degree of fusion of the sacral vertebrae varies among species; it is least complete in the pig. Even when fusion is total, the composition of the sacrum is betrayed by the number of foramina that mark both surfaces; the dorsal and the ventral branches of the sacral nerves issue separately through these. The junction of the ventral surface with the cranial extremity forms a lip known as the promontory (Figure 2–12/1); though often inconspicuous, it is a reference point in obstetrics.

The number of caudal vertebrae varies greatly, even within a single species. These vertebrae show a progressive simplification in form, and although the first few resemble miniature lumbar vertebrae, the middle and later members of the series are reduced to simple rods. In addition to the usual features, the more cranial vertebrae of some species provide protection to the main artery of the tail in the form of ventral (hemal) arches, separate small chevron (V-shaped) bones connected to the undersurfaces of the bodies, or paired ventral (hemal) processes (Figure 2–12, E).

The contours of the vertebral column vary with the posture, the species, and the breed. In general, the vertebrae from the caudal thoracic region to the tail head follow a more or less horizontal line. The more cranial thoracic vertebrae slope downward to reach the lowest point at the entrance to the chest, where an abrupt change in direction puts the spine on a course that ascends toward the head. The ventral inclination of the cranial thoracic vertebrae is masked in the live animal by the height of the spinous processes; indeed, in some species, the horse most notably, the spines are so long that the contour of this part of the back is raised to constitute the withers. Except toward the poll, the cervical vertebrae run at some distance from the dorsal skin. This is not apparent in the live subject, and in larger animals it may not be easy to determine, even on palpation. The greater part of the tail hangs down in large animals, but its posture is more variable in dogs and cats, being an expression of emotion in both species and influenced by breed in the former.

The Joints of the Vertebral Column

The vertebrae form two sets of joints: one cartilaginous, involving the direct connection of the vertebral bodies, the other synovial, existing between facets carried on the vertebral arches. In addition, certain long ligaments extend over many vertebrae. This pattern is modified in two regions; cranially, allowance is made for the free movement of the head, and in the pelvic region, sacral fusion occurs.

The two joints of the atlas are described first. The atlantooccipital joint (Figure 2–13) is formed between the condyles of the skull and the corresponding concavities of the atlas. Although the separate right and left articular surfaces converge ventrally, they do not always merge; despite this, a single synovial cavity generally exists. The synovial membrane attaches around the occipital and atlantal facets. It is strengthened externally by dorsal and ventral atlantooccipital membranes, which pass from the arches of the atlas to corresponding parts of the margin of the foramen magnum (see Figure 2–32/12), and by lesser lateral ligaments, which pass between the atlas and adjacent regions of the skull. Despite its odd character, the joint functions as a ginglymus: movement is virtually restricted to flexion and extension in the sagittal plane (the nodding movement that in ourselves conveys agreement).

Figure 2–13 Canine atlantooccipital joint, dorsal view; the dorsal arch of the atlas has been removed. 1, Skull; 2, atlantooccipital joint capsule; 3, wing of atlas; 3′, dorsal arch of atlas, resected; 4, atlantoaxial joint capsule; 5, axis; 5′, spine of axis, its overhanging cranial portion having been removed; 6, dens; 7, transverse ligament of atlas; 8, alar ligaments; 9, apical ligament of dens; 10, dorsal margin of foramen magnum.

The atlantoaxial joint is even more peculiar. The extensive articular surfaces of the ventral arch of the atlas and of the body and dens of the axis face into a single synovial cavity. The surfaces are so formed that only limited areas are in contact in any position of the head. This limitation of contact, together with the roomy capsule, allows some versatility of movement, although free excursion is confined to rotation about a longitudinal axis (the head-shaking movement that implies negation). The dorsal atlantoaxial ligament that joins adjacent parts of these vertebrae imposes little restraint. The dens of the axis, which occupies a potentially dangerous position in relation to the spinal cord, is secured by one or more ligaments that strap it to the adjacent part of the upper surface of the ventral atlantal arch and sometimes also to the occipital bone (as in the dog). It is rupture of these ligaments—or fracture of the dens itself—that allows the axis to strike against the cord and procure death in judicial hanging, according to traditional accounts (other forms of cervical fracture or dislocation may be at least as common).

A single description serves for the articulations of most other vertebrae. The intervertebral articulations combine symphyses between the bodies and synovial joints between the articular processes. The bodies of adjacent bones are connected through thick but flexible pads, the intervertebral disks, which make an appreciable contribution to the articulated column. They account for about 10% of its length in ungulates, about 16% in dogs, and about 25% in ourselves, which are proportions that are clearly correlated with different degrees of suppleness of the trunk. The disks are among the organs that most consistently show degenerative changes with advancing age; disk lesions are a common source of back trouble, long recognized in ourselves and in dogs, now also diagnosed in other domestic and even wild animals. Therefore, their structure has considerable importance, and it may be wise to stress that the details of anatomy and the nature of the troubles that may occur are not the same in ourselves as in quadrupeds.

Each disk consists of two parts, a nucleus pulposus and an anulus fibrosus (Figure 2–14). The nucleus occupies a slightly eccentric position. In the young animal, it consists of an unusual semifluid tissue derived from the embryonic notochord and retains some resemblance to this in structure. It is contained under pressure and escapes if afforded opportunity. The anulus fibrosus consists of encircling bundles of fibrous tissue that pass obliquely from one vertebra to the other, in most species merging with cartilage plates that cap the bones. The orientation of the fibers changes between successive lamellae, of which about a score exist. The distinction between anulus and nucleus is not always very clear, particularly in the larger species. Retention of the nucleus within the fibrous ring absorbs shock and spreads the compressive forces to which the column is subjected over a wider part of the vertebrae.

Figure 2–14 Bovine lumbar intervertebral disk. 1, Spinous process; 2, lamina; 3, synovial intervertebral joint; 4, articular process of adjacent vertebra; 5, vertebral canal with contents (spinal cord and meninges surrounded by epidural fat); 6, nucleus pulposus; 7, anulus fibrosus.

Insidious changes involving both nucleus and anulus commence relatively early in life. Fragmentation of the ring may allow the nucleus to escape, usually in the direction of the vertebral canal, where, directly or indirectly, it may press on the cord. Calcification of the nucleus diminishes the normal resilience and flexibility of the spine. Degenerative changes may affect any disk, but the effects are naturally likely to be most severe when they involve the disks at the most mobile regions; those of the neck and, in large animals, that at the lumbosacral junction are especially susceptible. Most thoracic disks are crossed dorsally by the intercapital ligaments that unite the heads of the right and left ribs (p. 43), and these are alleged to mitigate the effects of disk rupture at these levels.

The joints between the facets on the vertebral arches are conventional synovial joints. The nature and degree of mobility vary with the region and, to some extent, also with the species. In the cervical and cranial thoracic regions the joint surfaces are arranged tangential to the circumference of a circle centered in the vertebral body (see Figure 2–10); in these regions, rotation is possible in addition to the usual flexion and extension. In the caudal thoracic and lumbar regions the surfaces have a radial alignment, and movement is more or less restricted to the median plane. Movement is most free in the neck, where the articular surfaces are largest and the capsules most loose. The elastic interarcuate ligaments that fill the dorsal spaces between the arches of successive vertebrae may be regarded as accessory to these joints; their extent is inversely related to the width of the arches. In certain regions, interspinous and intertransverse ligaments also exist, but these are of less importance.

Three long ligaments extend along substantial portions of the column. A dorsal longitudinal ligament (Figure 2–15/7) runs along the floor of the vertebral canal from the axis to the sacrum. Narrow over the middle of each vertebral body, it widens where it crosses each intervertebral disk. A ventral longitudinal ligament follows the ventral aspect of the vertebrae from the midthoracic region to the sacrum; more cranially, its role is filled by the longus colli muscles. It also widens over and fuses with the intervertebral disks.

Figure 2–15 Ligaments of the vertebral column. Paramedian section of lumbar vertebrae of a dog; viewed from the left. 1, Supraspinous ligament; 2, spinous process; 3, interspinous ligament; 4, arch of vertebra; 5, interarcuate ligament; 6, intervertebral foramen; 7, dorsal longitudinal ligament; 8, ventral longitudinal ligament; 9, intervertebral disk.

A third (supraspinous) common ligament runs over (or to each side of) the summits of the spinous processes of the thoracic and lumbar vertebrae. It merges with the tendons of the epaxial muscles so completely that some dispute its independent existence. Except in the pig and cat, a cranial continuation of this ligament leaves the highest spines of the withers and runs by the shortest route to attach to the nuchal surface of the skull or, as in the dog, the spinous process of the axis (see Figure 2–8). This nuchal ligament runs close to the upper contour of the neck, and for most of its length, it is well separated from the more ventral course followed by the cervical vertebrae. Unlike the other long ligaments, it is elastic and thus able to accept much of the burden of the head when this is held high without interfering with the animal’s ability to lower the head to feed or drink from the ground. There is an obvious correlation between the strength of this ligament and the weight of the head and the length of the lever arm of the neck; the nuchal ligament is therefore much more powerfully developed in the larger species (see Figure 19–3), in which it is also more complicated in structure.

The Ribs and Sternum

The thoracic skeleton is completed by the ribs and sternum. The ribs (costae) are arranged in pairs and generally articulate with two successive vertebrae: the caudal one is that with the same numerical designation as the rib. Each rib consists of a bony dorsal part, the rib proper, and a cartilaginous ventral part, the costal cartilage (Figure 2–16, A). The two parts meet at a costochondral junction. The dorsal part of the rib articulates with the vertebral column, while the cartilage articulates with the sternum either directly, as do the first eight or so sternal or “true” ribs, or indirectly through connection of the cartilage with that in front, as do the asternal or “false” ribs. In this way, the cartilages of the asternal ribs combine to form the costal arch (Figure 2–17, A/6), the cranial boundary of the flank. The cartilage of the last rib may fail to make contact with its neighbor, and this rib is then said to be “floating.”

Figure 2–16 A, Left rib of a dog, caudal view. B, Left rib of a dog articulating with two vertebrae, lateral view. 1, Tubercle; 2, head; 3, neck; 4, angle; 5, body; 6, costochondral junction; 7, costal cartilage; 8, intervertebral disk; 9, vertebra of same number as rib.

Figure 2–17 A, Canine and B, equine sternum and costal cartilages, ventral and left lateral views. 1, Manubrium; 2, first rib; 3, sternebra; 4, costochondral junction; 5, xiphoid cartilage; 6, costal arch; 7, floating rib.

The dorsal extremity of the rib terminates in a rounded head that carries two facets, one for articulation with the body of each of the two vertebrae with which it is connected. These facets are separated by a rougher area (crest) that makes contact with the intervertebral disk and on most ribs also gives origin to the intercapital ligament. The head is joined to the body of the rib by a short constricted neck whose lower part carries a lateral tubercle. The tubercle bears a third articular facet, which meets that on the transverse process of the more caudal of the associated vertebrae (Figure 2–16, B).

The body of the rib begins beyond the tubercle. It is long, curved in its length, and usually laterally flattened, particularly in the larger species and toward the lower extremity. It is most strongly bent at a region known as the angle (Figure 2–16/4), where the lateral surface is roughened for the attachment of the iliocostalis. The cranial and caudal margins of the body are often sharply defined and give attachment to the intercostal muscles that fill the space between successive ribs. The caudal margin may also be grooved to give protection to the neurovascular bundle of the intercostal space.

The costal cartilage is flexible in the young animal, especially if it is long and thin, as in the dog. It becomes more rigid as calcification develops and increases with age. The cartilage either meets the bony rib at an angle (knee, genu) or is itself flexed cranioventrally some way beyond the costochondral junction.

Serial changes are obvious. The first rib is always relatively strong, short, and straight. Its cartilage is also stumpy and articulates with the sternum at a tight joint that fixes the rib; this allows it to act as a firm base toward which the other ribs may be drawn on inspiration. The succeeding ribs increase in length, in curvature, and in caudoventral inclination, most markedly over the caudal part of the thoracic wall, although the very last two or three may again be somewhat shorter. The three articular facets of the upper end approach and eventually merge on the ribs toward the end of the series. The cartilages of the sternal ribs are short and about as thick as the bony ribs; those of the asternal ribs are mostly slender and taper toward their ventral extremities.

The sternum is composed of three parts. The most cranial part, known as the manubrium (Figure 2–17/1), generally projects in front of the first ribs and may be palpated at the root of the neck. It is rodlike in the dog and cat but is laterally compressed in the larger animals. The body of the bone is composed of several segments (sternebrae), in youth joined by cartilage that is later replaced by bone. It is cylindrical in the dog, wide and flat in ruminants, and carries a ventral keel in the horse (Figure 2–17, B). Its dorsolateral margin bears a series of depressions in which the extremities of the costal cartilages are lodged. The more cranial of these depressions alternate with the sternebrae, and each receives a single cartilage; the more caudal depressions are crowded more closely together and may receive more than one cartilage. The caudal part of the sternum consists of flat (xiphoid) cartilage (Figure 2–17/5) that projects between the lower parts of the costal arches. It supports the most cranial part of the abdominal floor and gives attachment to the linea alba.

The Joints of the Thoracic Wall

Most ribs make two separate articulations with the vertebral column. The head participates in a ball-and-socket costovertebral joint of unusually restricted mobility. The joint cavity is divided into two compartments by the intercapital ligament (Figure 2–18/2), which arises from the interarticular crest. This ligament passes through the intervertebral foramen, crosses the floor of the vertebral canal, and ends by inserting on the corresponding region of the rib of the other side. In its passage, it detaches slips that anchor to the intervertebral disk and the adjacent parts of the vertebrae. It passes below the dorsal longitudinal ligament (Figure 2–18/6) and offers some protection against nuclear material from a ruptured disk protruding into the vertebral canal. An intercapital ligament is not found at the first costovertebral joint or at the last few. Additional short and tight ligaments support the joint dorsally and ventrally.

Figure 2–18 Costovertebral articulations; transverse section of the vertebral column of the dog (about T8). 1, Lamina of vertebra; 2, intercapital ligament; 3, tubercle of rib; 4, head of rib; 5, intervertebral disk; 6, dorsal longitudinal ligament; 7, costovertebral joint; 8, costotransverse joint covered by costotransverse ligament.

The costotransverse joint in which the tubercle participates is of the sliding variety. It is supported by a ligament that passes between the neck of the rib and the transverse process of the vertebra (Figure 2–18/8).

The costosternal joints are synovial joints of the pivot variety. The interchondral joints of the asternal ribs are syndesmoses of a rather elastic nature. The intersternal joints are mostly impermanent synchondroses, although in some species the manubrium articulates with the body at a synovial joint.

The movements possible at these joints are discussed with the actions of the muscles of the thoracic wall.

The Pelvic Girdle

Although the pelvic girdle is formally a part of the hindlimb skeleton, it seems more sensible to treat it here since it is fully integrated into the construction of the trunk. The girdle consists of symmetrical halves, the hip bones (ossa coxarum), which meet at the pelvic symphysis ventrally and form firm, though not rigid, articulations with the sacrum dorsally. When augmented by the sacrum and first few tail vertebrae, it forms a ring known as the bony pelvis around the pelvic cavity. The close association with the pelvic organs exposes the girdle to visceral influences of which those related to giving birth are most important; the form of the bony pelvis therefore reflects a compromise between these and the requirements of locomotion and posture.

Each hip bone is composed of three bones that develop from separate ossifications within a single cartilage plate. In the young animal, strips of cartilage demarcate the boundaries to allow for growth, but they disappear once growth is complete. It is therefore artificial to describe the three components—ilium, pubis, and ischium—as separate units; the practice can be justified only by its convenience in facilitating description. The ilium (Figure 2–19/1) is the craniodorsal part that extends obliquely forward from the hip joint to articulate with the sacrum. The pubis (Figure 2–19/6) extends medially from the joint to form the cranial part of the pelvic floor. The ischium (Figure 2–19/8) is more caudal and forms the larger part of the floor, although it also sends a branch to the joint. Both pubis and ischium participate in the symphysial joint in domestic species, although only the pubis does so in the human pelvis.

Figure 2–19 Canine hip bones in left lateral (A) and ventral (B) views. Dorsal (C) view of equine pelvis. The broken lines give the approximate extents of ilium, pubis, and ischium. 1, Wing of ilium; 2, ventral iliac spines; 2′, coxal tuber; 3, dorsal iliac spines; 3′, sacral tuber; 4, greater sciatic notch; 5, ischial spine; 6, pubis; 7, obturator foramen; 8, ischium; 9, ischial tuber; 10, lesser sciatic notch; 11, acetabulum; 12, pelvic symphysis; 13, ischial arch; 14, iliopubic eminence; 15, auricular articular surface; 16, sacrum.

The ilium consists of a cranial expansion or wing and a caudal shaft or body. The wing varies much among species; it is oblong with a more or less sagittal orientation in the dog and cat and is triangular and almost vertical in the horse and ruminants (see Figure 2–19). Its margin forms saliences, generally thickened, at certain points. Dorsally (dorsomedially in the larger species), it forms a sacral tuber; this is reduced to two low (cranial and caudal dorsal iliac) spines in the dog and cat (Figure 2–19/3) but is prominent in the large animals, in which it is close to the spinous processes of the vertebrae (Figure 2–19/3′). Ventrally (ventrolaterally in the larger species), the ilium forms a coxal tuber (Figure 2–19/2′,2); this is also reduced to low (cranial and caudal ventral iliac) spines in the carnivores but is prominent in large species, forming the point of the hip at the dorsocaudal corner of the flank (Figure 2–20, B/8). Including these projections, the margin of the wing is known as the iliac crest; thickened and convex in carnivores, it is thin and concave in large animals. Some of these features form important landmarks in the living animal.

Figure 2–20 Canine sacrotuberous ligament (A) and bovine sacrosciatic ligament (B), left lateral views. 1, Ilium; 2, sacrum; 3, caudal vertebra(e); 4, sacrotuberous ligament (in A), sacrosciatic ligament (in B); 5, ischial spine; 6, acetabulum; 7, ischial tuber; 8, coxal tuber; 9, sacral tuber; 10, greater sciatic foramen; 11, greater trochanter; 12, obturator foramen; 13, lesser sciatic foramen.

The lateral (dorsolateral) surface is excavated and largely given over to the origin of the gluteus medius, whose attachment may raise one or more quite prominent ridges. The medial (ventromedial) surface faces toward the body cavity. The ventral part gives origin to the iliacus, while more dorsally it bears the roughened auricular articular surface (see Figure 2–19, B/15) for the sacrum. The dorsal border of the wing is cut away at its junction with the shaft, forming the greater sciatic notch (incisura; see Figure 2–19/4), over which the sciatic nerve runs in passage to the hindlimb.

The shaft of the ilium is robust and columnar. Its caudal extremity contributes to the acetabulum, the deep cavity that receives the head of the femur. Its ventral border is marked by the low arcuate line that serves as part of the arbitrary boundary (“terminal line”) between the abdominal and pelvic cavities. Except in the dog, the line carries the psoas tubercle midway along its length; the psoas minor attaches here.

The pubis (Figure 2–19/6), essentially L-shaped, consists of cranial (acetabular) and caudal (symphysial) branches. The lateral end of the cranial branch contributes to the acetabulum and is known as the body. Its cranial edge, known as the pecten of the pubis, bears the iliopubic eminence and gives attachment to the abdominal muscles. Between them, the two branches account for about half the circumference of the obturator foramen (Figure 2–19/7), the large opening in the pelvic floor through which the obturator nerve emerges. The foramen is closed by muscle and membrane in the fresh state.

The ischium (Figure 2–19/8) consists of a horizontal plate extended cranially by symphysial and acetabular branches, one to each side of the obturator foramen. The extremity of the acetabular branch that contributes to the articular cup is known as the body. The body and the cranial part of this branch are surmounted by a crest, the ischial spine (Figure 2–19/5), which also extends onto the caudal part of the ilium. Marked by the origin of the gluteus profundus, it is relatively low in the dog and particularly high in ruminants. The caudolateral corner of the plate forms the ischial tuber (Figure 2–19/9); the border between this and the spine is indented by the lesser sciatic notch (Figure 2–19/10). The ischial tuber is a horizontal thickening in the dog, and a conspicuously triangular swelling in cattle. In most species it is subcutaneous, and it may be a visible landmark. The remaining part of the caudal border forms with its fellow the ischial arch, a notch that is broad and, except in the horse, shallow.

The acetabulum is a deep articular cup to which all three bones contribute; an additional small acetabular bone may be found in young animals. The acetabulum is contained by a prominent rim that is interrupted by a notch caudoventrally. It carries a lunate articular surface internally, but the depth of the cup is nonarticular and rough.

Species differences in the general form of the pelvic girdle are very pronounced. The ilium is most vertical in the larger and heavier species, which is a conformation that brings the sacroiliac joint, and therefore the weight of the trunk, more nearly above the hip joint (Figure 2–20, B). In smaller species, in which this consideration is of less importance, the ilium is very oblique (see Figure 2–1). This displaces the pelvic floor caudally relative to the vertebral column and increases the effectiveness of the abdominal muscles that flex the column in bounding gaits. Caudal displacement of the ischial tuber also increases the leverage that may be exerted by the hamstring muscles, the powerful extensors of the hip that arise here.

The dimensions of the girdle are most important in species that carry a single large offspring. They are of little significance in polytocous species (those that normally carry a litter), in which the full-term fetuses are relatively small. These aspects of pelvic conformation are discussed in later chapters.

The Joints and Ligaments of the Pelvic Girdle

The pelvic symphysis is a secondary cartilaginous joint that ossifies with advancing age. The process of ossification is irregular; it commences at different ages and advances at different rates, even in a single species. It is usually more precocious in onset and more advanced at any stage in the pubic than in the ischial part. It is sometimes asserted that in certain domestic species changes can be detected in the tissues of the symphysis (and sacroiliac joint) in advance of parturition. If this is so, and it is not universally accepted, these changes are minor in comparison with those that occur in guinea pigs and many other small animals at this time; in these, complete dissolution of the symphysis, which allows the two halves of the girdle to move apart to enlarge the birth passage, may occur.

The sacroiliac joints are curious in combining a synovial joint with an adjacent region of extensive fibrous union. The arrangement appears designed to combine firmness of attachment with some shock-absorbent capacity, for these joints are required to transmit the weight of the trunk to the hindlimbs when standing and the thrust of the limbs to the trunk in progression. The sacrum is wedged between the two halves of the pelvic girdle; each sacral wing carries an articular surface that is broadly flat (but irregular in detail) to match the corresponding iliac surface. The joint capsule is tight and is surrounded and supported by short fascicles of connective tissue that join adjacent parts of the two bones. It is a matter of preference whether certain longer sacroiliac ligaments, at a greater distance from the synovial articulation, are to be regarded as components of that joint or as independent structures. They may include long and short dorsal ligaments passing between the wing of the ilium and the spinous processes and other features of the sacrum. A ventral ligament offers more immediate support to the joint.

The sacrotuberous ligament (Figure 2–20/4) is of considerably greater interest. In the dog, it is a stout rounded cord extending between the caudolateral angle of the sacrum and the lateral part of the ischial tuber; no such ligament is present in the cat. In ungulates, it is better named the sacrosciatic ligament because it is expanded to a broad sheet that largely fills the space between the lateral border of the sacrum and the dorsal border of the ilium and ischium, which leaves open two foramina adjacent to the greater and lesser sciatic notches. The caudal edge is palpable in dogs and cattle (see p. 490 and p. 698).

The Cutaneous Muscle of the Trunk

The cutaneous muscle of the trunk (Figure 2–21) varies in relative thickness and extent but generally covers the lateral aspect of the thorax and abdomen with fascicles of a predominately horizontal course. It is contained within the superficial fascia and has as its main function tension and twitching of the skin. In some animals detachments are associated with the prepuce, and in horses and cattle a separate lamella covers the shoulder and arm regions. The innervation comes from the brachial plexus.

Figure 2–21 The cutaneous muscle of the dog.

The Muscles of the Vertebral Column

These can be separated into two divisions according to their position and innervation. The epaxial division (Figure 2–22, B/12) is placed dorsal to the line of the transverse processes of the vertebrae and receives its nerve supply from dorsal branches of the spinal nerves. The hypaxial division (Figure 2–22/14) lies ventral to the transverse processes and is supplied by the ventral branches of these nerves; it includes the muscles of the thoracic and abdominal walls in addition to those placed closely on the vertebrae. The thoracic and abdominal muscles are considered in later sections.

Figure 2–22 A, Trunk muscles of the dog, lateral view; the limbs have been removed. B, Epaxial (hatched) and hypaxial (stippled) muscles shown in a transverse section of the lumbar region. C, The three systems of epaxial muscles at the level of the thorax. 1, Coccygeus; 2, dorsal sacrocaudal; 3, levator ani; 4, external abdominal oblique; 5, its aponeurosis, pelvic tendon, and inguinal ligament; 5′, abdominal tendon; 6, vascular lacuna; 7, iliopsoas; 8, internal abdominal oblique; 9, wing of ilium; 10, acetabulum; 11, ischial tuber; 12, epaxial muscles; 13, lumbar vertebra—its transverse process appears as detached section; 14, hypaxial muscles; 15, psoas muscles; 16, transverses abdominis; 17, rectus abdominis; 18, flank fold; 19, iliocostalis system (crosshatched); 20, longissimus system (vertically hatched); 21, transversospinalis system (horizontally hatched); 22, thoracic vertebra and ribs; 23, peritoneum.

The Epaxial Muscles

These are numerous and complicated but fortunately do not require detailed description as they are rarely of clinical importance, except in the dog (p. 415). The major muscles are arranged in three parallel columns (Figure 2–22, C/19-21), which show some tendency to fuse over the loins and to split into additional units in the neck. They are extensors of the vertebral column, locally or more generally according to their extent, and are relatively more powerful in animals that make use of a bounding gait when traveling at speed (e.g., the dog).

The lateral column, the iliocostalis, arises from the ilium and transverse processes of the lumbar vertebrae and inserts on the more cranial lumbar vertebrae and ribs with, in most species, a weaker continuation into the neck. It is composed of many fascicles that overlap; for the most part they span about four vertebrae. Its lateral position also makes it effective in bending the trunk to the side (Figure 2–23, B/17).

Figure 2–23 A and B, Trunk muscles of the dog, deeper layers. 1, Longus capitis; 2, trachea; 3, esophagus; 4, splenius; 5, 6, serratus dorsalis cranialis and caudalis; 7, internal abdominal oblique; 8, its aponeurosis; 9, rectus abdominis; 10, caudal free border of internal abdominal oblique; 11, cremaster; 12, inguinal ligament; 12′, external abdominal oblique aponeurosis, cut and reflected; 13, fascia iliopsoas; 14, dorsal sacrocaudal muscles; 15, transversospinalis system; 15′, semispinalis capitis; 15″, spinalis et semispinalis; 16, longissimus system; 16′, longissimus capitis and cervicis; 16″, longissimus thoracis; 17, iliocostalis; 18, transversus abdominis; 19, transverse fascia.

The middle column, the longissimus (Figure 2–23/16), is strongest and can be followed into the neck, even to the head. Some of its more cranial parts are independent to a greater or lesser degree. The caudal attachments, which are the conventional origin, are from the ilium, the sacrum, and the mamillary processes, whereas the insertions are to the transverse processes and ribs. The fascicles thus pursue a cranial, lateral, and ventral course, and each bridges several vertebrae; the longest fascicles span the especially mobile thoracolumbar junction. Different parts may be designated longissimus lumborum, longissimus dorsi, longissimus cervicis, longissimus atlantis, and longissimus capitis, but usually the generic term is sufficient. The muscle tends to fuse with its medial and lateral neighbors in the lumbar region.

In addition to the more or less direct continuation, the cervical part of the longissimus is closely associated with the more superficial splenius (Figure 2–23, A/4). This passes from the highest spines of the withers and thoracolumbar fascia to the occipitomastoid region of the skull. It is covered by certain muscles of the thoracic girdle, especially the trapezius and rhomboideus.

The longissimus complex also includes certain small muscles passing between adjacent transverse processes as well as the dorsal (sacrocaudal) muscles of the tail (Figure 2–23/14); the latter are fleshy at their origin and are continued by tendons that run the length of the tail.

The medial column, the transversospinalis system (Figure 2–24/2), is the most complex, although the number of discrete units into which it may be divided varies among species. It lies on and between the medial parts of the vertebral arches and the spinous processes. Some fascicles run sagittally; others pursue a cranial, medial, and dorsal course from their caudal origin. The sagittal bundles include small units, often converted into ligaments, passing between adjacent spinous processes as well as larger units that span several vertebrae. The oblique bundles run from mamillary to spinous processes and may be distinguished by name according to whether they span one, two, three, or more joints. The longest fascicles are again concentrated at the middle, most mobile region of the back.

Figure 2–24 A and B, Trunk muscles of the dog, deepest layers. 1, Longus capitis; 2, transversospinalis system; 2′, multifidus; 2″, spinalis cervicis; 2′″, spinalis et semispinalis; 3, quadratus lumborum; 4, rectus abdominis; 5, transversus abdominis; 5′, its aponeurosis; 6, external intercostal muscles; 7, internal intercostal muscles; 8, rectus capitis ventralis; 9, longus colli; 10, psoas minor; 11, iliopsoas (psoas major and iliacus).

A number of specialized units bridge the joints between the axis, the atlas, and the skull and are responsible for the special movements in this region. Those of the dog are briefly described later (p. 415).

The Muscles of the Thoracic Wall

The muscles of the thoracic wall are primarily concerned with respiration. Most are inspiratory and enlarge the thoracic cavity, causing air to flow into the lungs. Some are expiratory and diminish the cavity, expelling air. They comprise muscles that fill the spaces between the ribs, certain small units placed lateral to the ribs, and, by far the most important, the diaphragm.

The intercostal muscles are theoretically arranged in three layers that correspond to the layers of the abdominal wall. The external intercostal muscles are outermost (Figure 2–24/6). Each of these muscles is confined to a single intercostal space in which its fibers run caudoventrally from an origin on one rib to a termination on the following rib. They fill the spaces from the upper ends to the costochondral junctions and sometimes beyond these but fail to reach the sternum. The parts between the cartilages are sometimes separately named. The internal intercostal muscles (Figure 2–24/7) are placed more deeply within the intercostal spaces and run cranioventrally, approximately perpendicular to the course of the external muscles. They do not occupy the most dorsal parts of the spaces, but, as if in compensation, they do reach the margin of the sternum. The third (subcostal) layer is so weak and so inconsistently developed that it may be ignored. The transversus thoracis is a triangular sheet that arises from and covers the dorsal surface of the sternum. The apex points cranially, and the muscle splits into slips that run caudolaterally to insert on the sternal ribs close to the costochondral junctions. It is morphologically the equivalent of the ventral part of the transversus abdominis.

Two muscles lie on the lateral surface of the thoracic wall. The rectus thoracis is a small quadrilateral sheet placed over the lower ends of the first four ribs in apparent continuation of the rectus abdominis. The serratus dorsalis (Figure 2–23, A/5,6) lies over the dorsal parts of the ribs. It takes origin from the fascia of the back and inserts on the ribs by a series of slips. The slips of the cranial part of the muscle slope caudoventrally, and those of the caudal part slope cranioventrally, which points to antagonistic functions. The two parts are sometimes quite widely separated. The scalenus, mentioned in the preceding section, has an attachment to the first rib; in some species it also passes quite extensively over the rib cage.

The diaphragm separates the thoracic and abdominal cavities. It is dome-shaped, being convex in all directions on its cranial surface, and bulges cranially under cover of the ribs to enlarge the abdomen at the expense of the thoracic cavity (Figures 2–2 and 2–25, A). It consists of a heart-shaped (trefoil-shaped in the dog) central tendon (Figure 2–25/7) and a muscular periphery that is divisible into portions that arise from the lumbar vertebrae, the caudal ribs, and the sternum.

Figure 2–25 A, Cranial view of the canine diaphragm. B, Lateral view of the canine thorax showing ribs and cranial extent of diaphragm in inspiration (broken lines) and expiration (solid lines). 1, Left crus; 2, right crus; 3, aorta; 4, esophagus; 5, attachment of caudal mediastinum to diaphragm; 6, sternal and costal parts of diaphragm; 7, tendinous center; 8, attachment of plica venae cavae; 9, caudal vena cava.

The central tendon is the most cranial part and forms the vertex. In the neutral position between full inspiration and full expiration, it reaches the level of the lower part of the sixth rib (or following space) and is thus only a little behind the plane of the olecranon in an animal standing square. Knowledge of this fact and of the line of the costal attachment is indispensable in appreciating the extent of the thoracic cavity (Figure 2–25, B).

The powerful lumbar portion of the peripheral muscle consists of left and right crura (Figure 2–25/1,2) that arise from the ventral aspect of the first three or four lumbar vertebrae by means of stout tendons. The right crus is considerably the larger, and it divides into three branches that radiate ventrally to join the central tendon. The left crus is undivided.

The much thinner costal part arises by serial digitations from the inner surfaces of the ribs and costal cartilages. The most caudal slip, which is also the most dorsal, arises close to the dorsal end of the last rib; those in front arise at successively more ventral levels, and the last costal digitation follows the cartilage of the eighth rib to the sternum. A final sternal slip arises from the dorsal surface of the sternum and runs dorsally to meet the tendon, which is thus bordered by muscle on all sides.

The diaphragm has three openings. The most dorsal, the aortic hiatus (Figure 2–25/3), is between the lumbar vertebrae and the crural tendons. It transmits the aorta, the azygous vein, and the thoracic duct. The esophageal hiatus (Figure 2–25/4) lies more ventrally, between the two medial divisions of the right crus. It transmits the esophagus, the dorsal and ventral vagal trunks that accompany the esophagus, and the vessels that supply it. The third opening, the caval foramen (Figure 2–25/9), lies within the central tendon, somewhat dorsal to the vertex and to the right of the median plane. It conveys the caudal vena cava and is of a rather different nature from the other openings because the adventitia of the vessel fuses with the tendon to leave no surrounding space. The margins of the other openings can slide over the structures passing through.

The diaphragm is supplied by the phrenic nerves formed from contributions by ventral branches of caudal cervical nerves (usually C5–C7). Despite the apparently involuntary nature of breathing, these are ordinary somatic nerves of mixed composition. The other muscles of the chest wall are supplied by intercostal nerves (ventral branches of thoracic spinal nerves).

The Muscles of the Abdominal Wall

The muscles of the abdominal wall are conveniently divided into ventrolateral and dorsal (sublumbar) groups (Figure 2–22, B). The first comprises the muscles of the flanks and abdominal floor; these muscles possess a particular importance because they are encountered and incised in almost all surgical approaches to abdominal organs. Most muscles of the second group properly belong to the girdle division of hindlimb musculature. They are included here because they constitute part of the body wall, namely, the roof of the abdomen to each side of the vertebral column.

The Ventrolateral Group

The intrinsic musculature of the flank comprises three broad fleshy sheets superimposed on each other with contrasting orientation of their fibers. Each is continued ventrally by an aponeurotic tendon that proceeds to a principal insertion within a fibrous cord, the linea alba, which runs in the ventral midline from the xiphoid cartilage to the cranial end of the pelvic symphysis (via the prepubic tendon). In so doing, the tendons ensheathe the fourth muscle, the rectus abdominis, which pursues a sagittal course within the abdominal floor directly to the side of the linea alba. The following account is of the basic arrangement. The details vary among species and may have surgical importance, especially in the small species (Figure 2–26; see also pp. 435–436).

Figure 2–26 Rectus sheath of the dog in transverse sections taken cranially (A) and caudal (B) to the umbilicus and near the pubis (C). 1, External abdominal oblique; 2, internal abdominal oblique; 3, transversus abdominis; 4, peritoneum; 5, cranial epigastric vessels; 5′, cranial superficial epigastric vessels; 6, rectus abdominis; 7, fat-filled falciform ligament; 8, linea alba; 9, caudal epigastric vessels; 9′, caudal superficial epigastric vessels; 10, internal lamina of rectus sheath; 11, external lamina of rectus sheath; 12, skin; 13, median ligament of the bladder.

The outermost external abdominal oblique muscle (Figure 2–22/4) arises from the lateral surfaces of the ribs and from the lumbar fascia. The majority of its fibers run caudoventrally; however, some radiation is present and allows the most dorsal bundles to follow a more horizontal course. The aponeurosis (Figure 2–22/5) that succeeds the fleshy part divides into two parts (tendons) before its insertion. The larger abdominal tendon terminates on the linea alba after passing ventral to the rectus muscle; the smaller pelvic tendon proceeds to attach on the fascia over the iliopsoas and on the pubic brim lateral to the insertion of the rectus (Figure 2–27/3′,4).

Figure 2–27 Inguinal canal and pelvic diaphragm of the dog, left lateral view. The external abdominal oblique muscle, present in A, has been removed in B. 1, External abdominal oblique; 2, internal abdominal oblique; 2′, free caudal edge of internal oblique, forming border of deep inguinal ring; 3, pelvic tendon of external oblique aponeurosis; 3′, caudal border of 3 (inguinal ligament) ending on 7; 3″, stump of external oblique aponeurosis reflected caudally (B); 4, abdominal tendon of external oblique aponeurosis; 4′, superficial inguinal ring; 5, cremaster derived from internal oblique; 6, vascular lacuna; 7, iliac fascia covering iliopsoas; 7′, iliopsoas; 8, acetabulum; 9, coccygeus; 10, levator ani.

The second muscle, the internal abdominal oblique (Figure 2–23/7), arises mainly from the coxal tuber (or the equivalent region of the ilium) but to lesser extents from the insertion of the pelvic tendon of the external oblique, the thoracolumbar fascia, and the tips of the lumbar transverse processes. This muscle fans out more obviously: its most caudal fascicles pass ventrocaudally, and although the next group runs more or less transversely in the plane of the coxal tuber, most pass ventrocranially. Some cranial fascicles insert directly on the last rib, but the bulk are continued by an aponeurosis (Figure 2–23/8) that passes ventral to the rectus to reach the linea alba. Toward the midline some interchange of fibers between the aponeuroses of the two oblique muscles usually occurs. The origin from the pelvic tendon allows the muscle a free caudal edge (Figure 2–23/10) that is mentioned again shortly in connection with the inguinal canal. A caudal slip (cremaster; Figure 2–23/11) detached from the internal oblique passes onto the spermatic cord (p. 191).

The deepest muscle of the flank, the transversus abdominis (Figure 2–24/5), arises from the inner surfaces of the last ribs and the transverse processes of the lumbar vertebrae. Its fibers run more or less transversely and are succeeded by an aponeurosis (Figure 2–24/5′) that passes dorsal to the rectus abdominis before terminating on the linea alba. This muscle does not extend caudal to the coxal tuber. The cauda part of the tendon passes ventral to the rectus so that the most caudal part of that muscle is left uncovered dorsally.

The fourth muscle, the rectus abdominis (Figure 2–23/9), forms a broad band to the side of the linea alba in the abdominal floor. It arises from the ventral surfaces of the rib cartilages and sternum and inserts on the pubic brim by means of a prepubic tendon. The fleshy part, which is widest about the middle of the abdomen, is divided into a series of segments by irregular transverse septa (tendinous intersections) that recall, even if they do not exactly reproduce, its polysegmental origin. The prepubic tendon serves as a common insertion for the abdominal muscles and the linea alba and may incorporate part of the tendons of origin of adductor (pectineus and gracilis) muscles of the thigh.

The rectus sheath (vagina musculi recti abdominis), the arrangement of the aponeurotic tendons of the flank muscles about the rectus abdominis, varies in detail among species. In the basic arrangement, the tendons of the two oblique muscles form a layer on the external (ventral) surface of the rectus, while that of the transversus lies against the internal surface; both layers merge with the linea alba to complete the enclosure (see Figure 2–26 and p. 436 for a fuller description of the rectus sheath in the dog).

The abdominal wall is perforated in the region of the groin by a passage known as the inguinal canal (Figures 2–27 and 21–5). Before or shortly after birth this transmits the testis in its descent toward the scrotum; in the adult male it contains the spermatic cord, consisting of the duct from the testis, and associated structures within an outpouching of the peritoneum. In both sexes, it also transmits the external pudendal artery and (usually) vein, efferent vessels from the superficial inguinal lymph nodes, and the genitofemoral nerve, which are all structures associated with the groin.

The term canal is misleading because it suggests a roomier passage than actually exists. The canal is a potential flat space between the fleshy part of the internal oblique on the one side and the pelvic tendon of the external oblique aponeurosis on the other (Figure 2–27/2,3). The walls are apposed and joined by areolar tissue except where the transmitted structures hold them apart. The slitlike abdominal entrance to the canal (the deep inguinal ring) lies along the free caudal edge of the internal oblique muscle (Figure 2–27/2′). The exit from the canal (the superficial inguinal ring; Figure 2–27/4′) is contained between the two divisions of the external oblique tendon. (The edges of the superficial inguinal ring are known as medial and lateral crura.) Species differences are mentioned in later chapters and may be of great importance since some explain why the escape of organs into and through the canal (inguinal hernia) occurs more readily in certain animals. Other differences are of immediate relevance to surgery in this area, most obviously in connection with castration, whether of the normal male or of one in which the testis has failed to descend and remains hidden within the abdomen or within the canal itself (a condition known as cryptorchidism).

The Muscles of the Pelvic Outlet

The pelvic outlet is closed about the terminal parts of the digestive and urogenital tracts by a portion of the body wall known as the perineum. The projection of the perineum on the skin outlines the perineal region, which has as its principal features the anus and the vulva (in the female, to which we principally refer here). Because the ventral part of the vulva falls below the level of the pelvic floor, it is usual to enlarge the concept of the perineal region to embrace the whole vulva. Very often the dorsocaudal part of the udder (in animals such as the cow) is also included. Several muscles and fasciae interlace in a node between the anus and the vulva and vestibule, and this formation is properly known as the perineal body or center; however, in clinical, especially obstetrical, literature the perineal body is frequently known simply, though incorrectly, as “the perineum.” The three concepts—perineum, perineal region, and perineal body—should be kept distinct. Another potential source of confusion exists. In human anatomy, the structures that occupy the pelvic outlet are said to form a “floor” to the pelvic cavity. In quadrupeds, the “floor” is provided by the pelvic girdle. The difference in posture not only affects the appropriate use of vernacular terms but, more important, also modifies the function of homologous structures. The principal component of the dorsal part of the perineum is the pelvic diaphragm, an arrangement of striated muscles contained between fasciae, which closes about the anorectal junction. A similar but less conspicuous arrangement in the ventral part of the perineum, the urogenital diaphragm, closes about the vestibule.

The pelvic diaphragm attaches laterally to the pelvic wall and spreads caudomedially to close about the anal canal. The term diaphragm aptly describes the human arrangement, which forms a basin in which the pelvic organs rest. It is less appropriate in domestic species, in which the “halves” of the diaphragm have more sagittal courses and converge more gently on the anus, which is the result of the relatively greater length of the pelvic girdle.

The more lateral of the two muscles of the diaphragm, the coccygeus (Figure 2–27/9), is essentially a muscle of the tail. Rhomboidal in outline, it arises from the ischial spine, crosses the sacrotuberous ligament medially, and inserts on and about the transverse processes of the first few tail vertebrae.

The medial muscle, the levator ani, is thinner and more extensive and runs more obliquely in a dorsocaudal direction; it is only partly covered by the coccygeus. The two muscles arise close together or by a common tendon in ungulates. In the dog, the levator has a more widely spread origin that continues from the iliac shaft over the cranial ramus of the pubis to follow the pelvic symphysis (Figure 2–27/10). The insertion is divided between the fascia and vertebrae of the tail (extending distal to the insertion of the coccygeus) and the fascia about the anus and external anal sphincter. The tail attachment predominates in carnivores, the anal one in ungulates, in which considerable exchange of fascicles with the anal sphincter and constrictor vestibuli muscles occurs.

The coccygeus flexes the tail laterally or, when acting in concert with its fellow, draws the tail ventrally to cover the perineum, an attitude familiar in the nervous dog. The action of the levator is best known from an electromyographic study in the goat, and it is possible that important species’ differences exist. In the goat it is active whenever the intraabdominal pressure is raised, presumably to oppose the tendency to displace pelvic organs caudally. Although also involved in other visceral functions, it has a very definite relationship to defecation; it is active before the event (when it may fix the position of the anus against the contraction of the smooth muscle of the colon), becomes inactive during the event, and regains activity following the event (when it may restore the parts to their resting positions). The jerky movements of the dog’s tail after defecation are probably evidence of levator activity in this species. Both muscles are supplied by ventral branches of the sacral nerves.

The smaller urogenital diaphragm (membrana perinei) contains more slender muscles, which are more appropriately described later with the reproductive organs. The fascia of the urogenital diaphragm attaches to the ischial arch and curves cranially, dorsally, and medially to blend with the ventral edge of the pelvic diaphragm and embrace the vestibule. It helps anchor the reproductive tract against a forward drag when the pregnant uterus sinks within the abdomen and against a backward displacement during parturition.

It may now be evident that to each side there is a space that is enclosed by the pelvic girdle but excluded from the pelvic cavity by the pelvic diaphragm. This space is pyramidal and has a cranial apex, a lateral wall furnished by the ischial tuber and sacrotuberous ligament, a medial wall furnished by the pelvic diaphragm, a ventral wall furnished by the pelvic floor, and a base directed toward the skin. It is appropriately known as the ischiorectal fossa and is normally occupied by fat (see Figure 29–10/12). When this fat is depleted, a pronounced sinking of the skin to the side of the anus is apparent (except in the horse and pig, in which the vertebral head of the semimembranosus covers the region).

The Skull of the Dog

This initial account is of the skull of an adult dog of average (mesaticephalic) conformation, neither short-headed (brachycephalic) like a Pekingese nor long-headed (dolichocephalic) like a Borzoi. Some salient breed differences are mentioned later (p. 374).

In the dorsal view (Figure 2–30), the ovoid cranium meets the bones of the face where the zygomatic processes (Figure 2–30/4′) of the frontal bones project laterally to form the dorsocaudal parts of the orbital walls. The caudal extremity of the cranium is marked by the external occipital protuberance in the midline; its demarcation from the caudal (nuchal) surface is completed by the nuchal crests that extend laterally to each side. The median sagittal crest that extends forward from the occipital protuberance is most prominent in robust, well-muscled animals. All these features are easily palpated in life. The dorsal and lateral surfaces of each half of the cranium blend in a continuous and slightly roughened surface from which the temporalis muscle arises. Rostral to the zygomatic processes of the frontal bones the dorsal surface of the skull dips, sometimes quite markedly, before continuing as the straight and narrow dorsum of the nose. This ends at the wide nasal aperture beyond which the bony skull is prolonged by pliant nasal cartilages.

Figure 2–30 Dorsal view of canine skull. 1, Nasal aperture; 2, infraorbital foramen; 2′, maxillary foramen; 3, fossa for lacrimal sac; 4, orbit; 4′, zygomatic process of frontal bone; 5, zygomatic arch; 6, external sagittal crest; 7, nuchal crest; 8, external occipital protuberance, 9, cranium.

The orbit is the most prominent feature of the lateral view (Figure 2–31). Behind the orbit, the dorsolateral part of the braincase forms the wall of the temporal fossa (Figure 2–31/16). The ventrolateral part is more complicated and presents the zygomatic arch and ear regions. The zygomatic arch (Figure 2–31/15) springs free from the braincase and, bowing laterally, passes below the orbit to rejoin the facial part of the skull. It is formed by two bones, the squamous temporal and zygomatic, which meet at an overlapping suture. The ventral surface of the caudal part of this arch carries the articular surface for the mandible, shaped as a transverse gutter in this species; the articular area continues caudal to this onto the rostral surface of a ventral projection, the retroarticular process (Figure 2–31/6). The large, smooth dome of the tympanic bulla (Figure 2–31/9) (enclosing part of the cavity of the middle ear) and the rough mastoid process lie behind the retroarticular process. Three openings are present in this region of the skull: the retroarticular foramen emits a major vein draining the cranial cavity, the stylomastoid foramen gives passage to the facial nerve, and the external acoustic meatus is, in the fresh state, closed by a membrane (eardrum) that separates the canal of the external ear from the cavity of the middle ear. The paracondylar process (Figure 2–31/11) is conspicuous at the caudal limit of the skull.

Figure 2–31 Lateral view of canine skull. 1, Orbital ligament (inset); 2, infraorbital foramen; 3, orbit; 4, pterygopalatine fossa; 5, optic canal, orbital fissure, and rostral alar foramen; 6, retroarticular process; 7, retroarticular foramen; 8, external acoustic meatus; 9, tympanic bulla; 10, stylomastoid foramen; 11, paracondylar process; 12, occipital condyle; 13, nuchal surface; 14, mastoid process; 15, zygomatic arch; 16, temporal fossa; 17, nuchal crest.

The orbit is funnel shaped, and in the macerated state its walls are very incomplete. In life the orbital rim is completed by a ligament (Figure 2–31/1) that connects the zygomatic process of the frontal bone to the zygomatic arch. Ventrally the orbital cavity is continuous with the pterygopalatine fossa (Figure 2–31/4), but in the fresh state these regions are separated by the periorbita, a dense fascial sheet that completes the definition of the orbit. Two groups of foramina are visible in this region. The caudal group (Figure 2–31/5) comprises the optic canal, orbital fissure, and rostral alar foramen. The optic opening, placed at the apex of the conical orbital cavity, is the portal of entry of the optic nerve. The more ventral orbital fissure transmits the nerves (ophthalmic, oculomotor, trochlear, and abducent) that supply ancillary structures of the eye and the external ophthalmic vein. Most ventrally the rostral alar foramen provides a common opening for the maxillary nerve, passing from the cranial cavity, and the maxillary artery, which transverses a canal (alar canal) in the sphenoid bone.

The rostral group of foramina comprises the maxillary, sphenopalatine, and caudal palatine foramina. The maxillary foramen (Figure 2–30/2′) leads to the infraorbital canal, the sphenopalatine foramen to the nasal cavity, and the caudal palatine to the palatine canal, which emerges on the hard palate; each opening conveys like-named branches of the maxillary artery and nerve. More dorsally the rostral orbital wall contains the lacrimal fossa for the lacrimal sac (Figure 2–30/3). An opening in the depth of the fossa leads to a passage that conveys the nasolacrimal (tear) duct to the nose.

The infraorbital foramen (Figure 2–30/2) is the most prominent feature of the lateral aspect of the face and is easily palpable in the live animal; it is the site of emergence of the infraorbital nerve, which continues from the maxillary nerve through the infraorbital canal. Toward the alveolar margin the facial skeleton is molded over the roots of the teeth, most especially over the large root of the canine tooth.

In the ventral view (Figure 2–32), three regions of the skull are distinct: the base of the cranium, the choanal region where the nasal cavities open into the pharynx, and the hard palate. The first shows at its caudal limit the ovoid, obliquely oriented occipital condyles that flank the foramen magnum (Figure 2–32/12) through which the spinal cord connects with the brain. Rostral to this the median area is generally flat, although midway along its length, tubercles are present for the attachment of muscles that flex the head on the neck. The tympanic bulla and paracondylar process occupy much space to each side. The medial aspect of the bulla (Figure 2–32/7) meets the occipital bone, and this fusion separates two openings that are confluent in some other species (e.g., horse; see Figure 2–37), namely, the more caudal jugular foramen and the more rostral foramen lacerum (Figure 2–32/8,6). The glossopharyngeal, vagus, and accessory nerves emerge through the jugular foramen together with a large vein draining the interior of the cranium. Between the jugular foramen and the condyle is the hypoglossal canal, which transmits the hypoglossal nerve.

Figure 2–32 Ventral view of canine skull. 1, Palatine fissure; 2, hard palate; 3, choanal region; 4, oval foramen; 5, base of cranium; 6, foramen lacerum; 7, tympanic bulla; 8, jugular foramen; 9, paracondylar process; 10, hypoglossal canal; 11, occipital condyle; 12, foramen magnum.

Lateral to the foramen lacerum, small fissures exist for the exit of the chorda tympani (a branch of the facial nerve) and for the communication of the cartilaginous auditory tube with the cavity of the middle ear. Rostral to these is the prominent oval foramen (Figure 2–32/4), through which the mandibular nerve emerges.

The openings (choanae) that lead from the nasal cavities to the nasopharynx are the main features of the middle part of the ventral aspect. The choanal region is bounded dorsally by the floor of the cranium and laterally by the thin plates of bone whose outer surfaces were earlier noted as forming the medial walls of the pterygopalatine fossae. The soft palate, which arises from the free margin of the hard palate, in life provides the floor of the space—essentially the first part of the nasopharynx—enclosed by these formations. The palate, which lies rostral to this, is broad behind and narrower in front. It is margined by the alveoli or sockets in which the upper teeth are implanted. Toward its rostral extremity, it is perforated by the large bilateral palatine fissures. Several smaller foramina toward the caudal extremity of the palate are rostral openings of the palatine canal.

The nuchal surface (Figure 2–31/13), broadly triangular, is limited dorsally by the external occipital protuberance and the nuchal crests. Its lower part presents the foramen magnum, the occipital condyles, and the paracondylar processes. The remainder of the surface is roughened for the attachment of dorsal muscles of the neck.

The apex of the skull is formed by the nasal aperture situated dorsal to the rostral extremities of the jaws that carry the incisor teeth.

The cavities of the skull are described with the respiratory system (Chapter 4), central nervous system (Chapter 8), and ear (Chapter 9).

The lower jaw or mandible comprises two parts (Figure 2–33). In the dog these are firmly but not rigidly united by the connective tissues of the mandibular symphysis. Each half is divided between a body, or horizontal part, and a ramus, or vertical part. The body carries the alveoli of the lower teeth and is laterally compressed. Except at its rostral extremity, it diverges from its fellow to bound an intermandibular space. Toward its rostral extremity the lateral surface presents several mental foramina, one generally much larger than the rest; through these emerge the mental branches of the inferior alveolar nerve and vessels. The ramus (Figure 2–33/2) is wider but less robust. Its dorsal extremity ends in the high recurved coronoid process, which projects into the temporal fossa and gives attachment to the temporalis muscle, and the lower and more caudal condylar process (Figure 2–33/3), which carries an articular head shaped like a portion of a truncated cone. The lower part of the caudal margin of the ramus carries the projecting angular process that enlarges the areas of attachment of the masseter and medial pterygoid muscles. The lateral surface is scooped out to provide a roughened depression where the masseter inserts. The medial surface gives insertion to the pterygoid muscles and also presents the large mandibular foramen (Figure 2–33/7), where the inferior alveolar vessels and nerve enter the bone.

Figure 2–33 Lateral (A) and medial (B) views of the left half of the canine mandible. 1, Coronoid process; 2, vertical part (ramus); 3, condylar process; 4, angular process; 5, horizontal part (body); 6, mental foramina; 7, mandibular foramen; 8, symphysial surface.

The hyoid apparatus consists of a series of bony rods, jointed together and forming a means of suspending the tongue and larynx from the skull. The names given to the several parts are shown in Figure 2–34, which illustrates their arrangement and the attachment of the apparatus as a whole to the temporal region of the skull. The transversely placed basihyoid may be palpated within the intermandibular space; other parts are palpable—indeed their positions are visible—when the walls of the pharynx are inspected through the mouth.

Figure 2–34 Hyoid apparatus and larynx suspended from the temporal region of a canine skull. 1, External acoustic meatus; 2, tympanic bulla; 3, stylohyoid; 4, epihyoid; 5, ceratohyoid; 6, basihyoid; 7, thyrohyoid; 8, epiglottic cartilage; 9, thyroid cartilage; 10, cricoid cartilage.

Some Comparative Features of the Skull

When equipped with the mandible the skull of the cat (Figure 2–35) appears globular. Several features combine to create this conformation: the rounded cranial capsule, surmounted by a short, often weak sagittal crest, and corresponding closely to the contours of the brain; the very salient convex zygomatic arches; and the relative shortness of the face, which may account for as little as 20% of the total length. Breed differences are more pronounced than sometimes supposed. The skulls of Siamese and similar cats have much longer faces, which often blend smoothly with the cranium without any break (stop) in the dorsal contour. In contrasting types, for example, the Persian, the face is short and shallow and the stop is prominent.

Figure 2–35 Feline skull with mandible. 1, Infraorbital foramen; 2, orbit; 3, zygomatic arch; 4, mental foramen; 5, temporomandibular joint; 5′, angular process of mandible; 6, external acoustic meatus; 6′, tympanic bulla; 7, occipital condyle; 8, nuchal crest; 9, sagittal crest; C, canine tooth; P⁴, upper fourth premolar.

The orbital region is distinctive. The orbits are large, face more directly forward than in the dog, and have more complete bony margins. The frontal process of the zygomatic bone and the zygomatic process of the frontal bone leave only a small gap in the ovoid margin to be closed by the orbital ligament. The zygomatic arch is surprisingly strong where it contributes to the orbital rim. The infraorbital foramen is placed close to the rostroventral part of the orbit, where it may be palpated.

On the ventral aspect, the hard palate is short, wide, and carries alveoli for only four cheek teeth. That for the largest (P⁴) of these teeth is located dangerously close to the orbit, which may become involved in a spreading alveolar abscess. Caudally, the deep gutter of the temporomandibular articulation is bounded by a prominent retroarticular process. The very large tympanic bulla is so salient that it may be palpated between the caudal part of the zygomatic arch and the wing of the atlas.

As in the dog, the halves of the mandible do not fuse, even in old age, and a small degree of movement is allowed at the mandibular symphysis. Each half carries sockets for only three cheek teeth.

The equine skull (Figure 2–36) is characterized by a relatively long face, a feature that develops further with increasing size; it is therefore more pronounced in mature than in juvenile animals and in large than in small breeds. The cranium is relatively narrow and generally not unlike that of the dog. The external sagittal crest is weaker. The forehead is wide between the origins of the zygomatic processes of the frontal bones, which bend ventrally to join the zygomatic arches.

Figure 2–36 A, Equine skull, and B, equine mandible. 1, Incisive bone; 2, nasoincisive notch; 3, nasal bone; 4, infraorbital foramen; 4′, cheek teeth; 5, facial crest; 6, hamulus of pterygoid bone; 7, zygomatic arch; 8, retroarticular process; 9, external acoustic meatus; 10, paracondylar process; 11, occipital condyle; 12, horizontal part (body) of mandible; 12′, mental foramen; 12″, vascular notch; 13, vertical part (ramus) of mandible; 13′, coronoid process; 13″, mandibular foramen; I, incisors; C, canine tooth (present only in the male).

The zygomatic arch (Figure 2–36/7) is conspicuously strong, even without taking into account the extra support it obtains from the zygomatic process connecting it with the frontal bone. It is not bowed laterally to any extent and carries a rather complicated articular surface on its caudoventral aspect; this comprises a rostral tuber, an intermediate fossa, and a salient retroarticular process (Figure 2–36/8). The orbit faces almost laterally and has a complete bony rim. A large maxillary tuberosity appears to continue the alveolar process directly. The zygomatic arch is continued rostrally, beyond the orbit, as a prominent ridge on the lateral surface of the face. This ridge, the facial crest (Figure 2–36/5), runs parallel to the dorsal contour of the nose and ends above a septum between the alveoli of the third and fourth cheek teeth in the adult.

A deep (nasoincisive) notch separates the pointed nasal bone from the incisive bone (Figure 2–36/1,2,3). This notch and the rostral end of the facial crest are both very easily identified landmarks; they are used as guides to the position of the infraorbital foramen, which lies a little caudal to the middle of the connecting line (Figure 2–36/4).

The features visible on the ventral view lie more or less on one level. The caudal part of this surface is distinguished by the large and very salient paracondylar processes (Figure 2–36/10) and the jagged outlines of the large openings to each side of the occipital bone. Each opening results from the failure of the temporal bone to reach the lateral margin of the occipital bone, which permits the confluence of several foramina that are distinct in the dog. The caudal part is the equivalent of the jugular foramen; the cranial part (foramen lacerum) combines the oval and carotid foramina (Figure 2–37/7,6). In life the greater part of the large opening is occluded by membrane that leaves barely sufficient passage for the various nerves and vessels. The tympanic bulla is not prominent, but styloid (for the hyoid apparatus) and muscular processes of the temporal bone are well developed.

Figure 2–37 Left caudolateral parts of the base of the equine (A) and canine (B) cranium, showing portions of the occipital (O), sphenoid (S), and temporal (T) bones; ventral view (schematic). 1, Foramen magnum; 2, occipital condyle; 3, hypoglossal canal; 4, jugular foramen; 5, foramen lacerum; 5′, petrooccipital suture; 6, carotid canal; 6′, carotid notches; 7, oval foramen; 7′, oval notch.

The choanae lie almost in the plane of the hard palate. The vertical plate of bone that separates the choanal from the pterygopalatine region carries a prominent hamular process (Figure 2–36/6). The palate is flat and unremarkable. The greater part of its margin is occupied by the alveoli of the incisor and cheek teeth.

A well-marked external occipital protuberance is present on the nuchal surface, midway between the nuchal crest and the dorsal margin of the foramen magnum.

The mandible is massive, and its right and left halves diverge at a relatively small angle (Figure 2–36, B). The symphysis becomes obliterated quite early, usually about 2 years after birth. The lower margin carries a prominent vascular notch where the facial vessels wind onto the face (Figure 2–36/12″). The ramus is high, the coronoid process projects far into the temporal fossa, and the articular process carries the ovoid articular surface well above the occlusal plane of the cheek teeth.

The parts of the hyoid apparatus (see Figure 4–8) are of different proportions to their counterparts in the dog and are laterally compressed. A substantial lingual process projects from the basihyoid into the root of the tongue.

The bovine skull (Figure 2–38) is relatively short and wide: its general form is pyramidal. Cornual (horn) processes project from the frontal bones of horned breeds where the dorsal, lateral, and nuchal surfaces meet; their size and direction vary greatly with breed, age, and sex. The very wide and flat frontal region is bounded by a prominent temporal line that overhangs the deep temporal fossa and confines this to the lateral aspect of the skull. The forehead continues smoothly into the dorsal contour of the nose.

Figure 2–38 Bovine skull with mandible. 1, Incisive bone; 2, mental foramen; 3, infraorbital foramen; 4, facial tuberosity; 5, nasal bone; 6, orbit; 7, frontal bone; 7′, horn surrounding cornual process of frontal bone; 7″, temporal line; 8, temporal fossa; 9, zygomatic arch; 10, external acoustic meatus; 10′, tympanic bulla; 11, paracondylar process; 12, occipital condyle; I, incisors; C, canine tooth, incorporated in the row of incisors.

The principal features of the lateral aspect are the confinement of the temporal fossa and the elevation of the orbital rim above its surroundings. The rim is complete and is formed by the meeting of processes from the zygomatic and frontal bones in its caudal part. There is no facial crest, only a discrete facial tuberosity from which the rostral part of the masseter arises. The infraorbital foramen is directly above the first cheek tooth, rather low toward the palate.

The ventral surface is very uneven, and the cranial base is located in a considerably more dorsal plane than the palate. The temporal and occipital bones are separated by a narrow fissure, which is an arrangement intermediate between the suture of the dog and the wide opening of the horse and pig. The tympanic bulla is prominent and laterally compressed. The choanae are separated by the caudal prolongation of the ventral part of the nasal septum and are enclosed laterally by very extensive plates of bone. The palate, long and narrow, is bounded by high alveolar processes. Of course, no alveoli are present for incisor or canine teeth, which are lacking in the upper jaws of ruminants.

The mandibular symphysis ossifies late, if at all, in ruminants. In general, the mandible is weaker than that of the horse, which is a feature very apparent in the body of the bone with its gently convex ventral border. The coronoid process is high and caudally inflected. The articular surface is concave and widened laterally.

The few remarks necessary regarding the skulls of the small ruminants and pig are found on pages 646 and 752, respectively.

The Joints of the Head

The articulations between the skull and mandible (temporomandibular joints) and that between the halves of the mandible (mandibular symphysis) are appropriately considered in the following chapter (p. 112) because the teeth, the muscles of mastication, and the joints form a single functional complex.

The Trigeminal Musculature

The muscles of mastication constitute the greater part of the musculature supplied by the mandibular division of the trigeminal nerve, the motor nerve to the first pharyngeal arch. They are described in the chapter on the digestive system (p. 113). The same chapter deals with the digastricus—a composite muscle to which the mandibular field makes a contribution; the mylohyoideus (p. 105), which slings the tongue between the lower jaws; and one (tensor veli palatini) of the muscles of the soft palate (p. 119). The tensor tympani is considered with the middle ear (p. 346).

The Facial Musculature

The musculature supplied by the facial nerve, the nerve of the second pharyngeal arch, is resolvable into two divisions. The superficial division comprises the cutaneous muscle of the head and neck in addition to many small units that control the posture of the lips, cheeks, nostrils, eyelids, and external ears. The deep division is rather scattered but includes some muscles associated with the hyoid apparatus, a contribution to the digastricus (p. 114), and the stapedius (p. 348) of the middle ear.

The Superficial Division

The muscles of this division are conjectured to have their source in an ancestral deep sphincter muscle of the neck, which may be envisaged as arranged in three incomplete overlapping layers. The outermost layer, consisting of transversely disposed fascicles, is reduced to insignificance or is entirely lacking in domestic mammals. A remnant (sphincter colli) survives in the dog. A more substantial portion of the middle layer commonly persists in the form of a sheet of longitudinally disposed fibers that covers the ventral part of the face and extends onto the neck, even reaching the nape in the dog. It is known as the platysma. Detached slips are believed to provide the small muscles that attach to the caudal aspect of the external ear.

The third and deepest layer is again transverse. Although little of it remains in sheet form, it is believed to be the origin of the many discrete muscles of the mammalian face. These are extremely variable among species, but fortunately, few units, and even fewer differences, require detailed notice. Because of their effect on the appearance of the face, they are collectively known as the muscles of facial expression or mimetic musculature.

The principal muscles of the lips and cheeks are the buccinator, orbicularis oris, caninus, levator nasolabialis, levator labii superioris, and depressor labii inferioris (Figures 2–39 and 11–6). The buccinator (Figure 2–39/4) passes between the margins of the upper and lower jaws and is partly covered by the masseter. It forms the basis of the cheek and acts in opposition to the tongue, preventing food from collecting in the vestibule by returning it to the central cavity of the mouth. The buccal salivary glands are scattered among its fascicles, and discharge of their secretion into the mouth may be assisted by contraction of the muscle. The orbicularis oris (Figure 2–39/1) surrounds the mouth opening, where it is closely attached to the skin and mucosa of the lips. It closes the opening of the mouth by pursing the lips and is important in sucking. The caninus (Figure 2–39/2) arises ventral to the infraorbital foramen and radiates into the wing of the nostril and the upper lip. It dilates the nostril and elevates the corner of the mouth in the snarling gesture, especially in the dog. The levator nasolabialis (Figure 2–39/5) arises over the dorsum of the nose and inserts partly on the wing of the nostril and partly into the lateral part of the upper lip. It is able to dilate the nostril and to elevate and retract the upper lip. The medial part of the upper lip is elevated by the separate levator labii superioris (Figure 2–39/6). This muscle arises on the lateral aspect of the face and runs dorsorostrally to form with its fellow a common tendon that descends into the lip between the nostrils. A special depressor labii inferioris is present in the lower lip of certain species (excluding the dog and cat). It appears to be a detachment from the buccinator muscle. Other muscles associated with the lips and nostrils do not merit specific mention, although some are identified in various illustrations.

Figure 2–39 Superficial muscles of the equine head. The cutaneous muscle has been removed. 1, Orbicularis oris; 2, caninus; 3, depressor labii inferioris; 4, buccinator; 5, levator nasolabialis; 6, levator labii superioris; 7, orbicularis oculi; 7′, levator anguli oculi medialis; 8, temporalis; 9, occipitomandibular part of digastricus; 10, masseter.

The muscles of the eyelids include one, the levator palpebrae superioris, that is clearly foreign to the facial group because it arises within the orbit and is supplied by the oculomotor nerve. It is described on page 342. The muscles of the lids that are supplied by the facial nerve include a sphincter—the orbicularis oculi (Figure 2–39/7)—that surrounds the palpebral fissure, the opening between the lids. It is anchored at the medial and lateral commissures and therefore narrows the opening to a horizontal slit when it contracts. Other muscles are present to raise the upper (levator anguli oculi) lid and to depress the lower (malaris) lid, enlarging the eye opening.

The muscles of the external ear are especially numerous but of little account individually. A caudal group has already been mentioned. Others converge on the auricle—the skin-covered cartilaginous ear “trumpet”—from medial, rostral, and lateral directions; they lie between the skin and the temporalis muscle and skull and form a thin, incomplete sheet that includes a (scutiform) cartilage plate. The scattered origins and precisely located insertions provide for displacement and rotation of the ear in all directions. One, the parotidoauricularis, is of somewhat greater importance because it is encountered in the operation for drainage of infections of the external ear of the dog (p. 399). As its name suggests, it arises from the fascia over the parotid gland and approaches the auricle from the ventrolateral direction.

Besides the individual functions mentioned or implied in the preceding paragraphs, these muscles have a collective function in communication, mainly within the species but also between species. Human observers can intuitively, or as the result of experience, interpret many facial gestures of animals: one need only recall the hangdog expression of submission, the evident threat conveyed by snarling or laying back the ears, or the quizzical look a dog may adopt. The analysis of the more subtle expressions in terms of specific muscle activity is not yet possible for domestic species.

Paralysis of these muscles is not uncommon after damage to the facial nerve. Since different groups are supplied by branches of the nerve that arise at different levels, the particular pattern of distortions can be a valuable pointer to the location of the nerve lesion (p. 318).

The Muscles of the Pharynx and Soft Palate

These are considered beginning on page 116.

The Muscles of the Larynx

These are considered beginning on page 153.

The External Muscles of the Eyeball

These are considered beginning on page 341.

The Muscles of the Tongue

These are considered beginning on page 104.

The Muscles of the Ventral Part of the Neck

The neck connects the head with the trunk and is usually distinguished by its relatively slender construction, although this is hardly true of the pig. It has a generally cylindrical form in the dog and cat but is quite obviously compressed from side to side in the larger animals, in which it deepens considerably toward its junction with the thorax (Figure 2–40). The core structures of the neck—the cervical vertebrae and the muscles closely applied to them—were described with the trunk (p. 47). Certain superficial muscles are considered under the heading of girdle muscles of the forelimb (p. 82). The present section is therefore concerned only with the ventral part of the neck, a region of considerable clinical importance on account of the numerous visceral, vascular, and nervous structures that traverse it en route between the head and thorax.

Figure 2–40 Transverse section of the bovine neck. 1, Rhomboideus; 2, trapezius; 3, nuchal ligament; 4, splenius; 5, omotransversarius; 6, vertebra; 7, longus colli; 8, brachiocephalicus; 9, external jugular vein in jugular groove; 10, 10′, sternocephalicus, mandibular, and mastoid parts; 11, combined sternohyoideus and sternothyroideus; 12, trachea; 13, esophagus (ventral to it, nerves, blood vessels, and thymus).

These structures, with the important exception of the external jugular veins (Figure 2–40/9), occupy a central visceral space. The roof of this space is provided by the muscles immediately ventral to the vertebrae, namely the longus colli, longus capitis, rectus capitis ventralis, and scalenus (p. 48). The side and ventral walls blend together and are provided by thinner muscles disposed with a sagittal course and joined by stout fasciae.

The cervical part of the cutaneous muscle (m. cutaneous colli) is unimportant in the dog and cat. It is much better developed in the ungulates, in which it radiates from a stout origin on the manubrium of the sternum; it thins as it passes cranially and laterally and eventually fades away. In the horse, the cutaneous muscle provides a relatively thick cover to the caudal third or so of the jugular groove.

The straplike sternocephalicus (Figure 2–41/2) is the most ventral of the other muscles. It also arises from the manubrium and is first pressed against its fellow. As it ascends the neck, however, it diverges laterally toward its insertion, which varies among species but includes one or the other (or both) of the angle of the mandible and the mastoid process of the skull. The divergence of the right and left muscles exposes the upper part of the trachea to palpation through the skin, although a very thin layer of deeper muscle still intervenes. The sternocephalicus is supplied by the ventral branch of the accessory nerve. Unilateral contraction draws the head and neck to that side. Bilateral contraction flexes the head and neck ventrally. In species with a mandibular insertion the sternocephalicus may assist in opening the mouth.

Figure 2–41 Ventral muscles of the canine neck and thorax. 1, Combined sternohyoideus and sternothyroideus; 2, sternocephalicus; 3, 3′, brachiocephalicus: cleidocervicalis, cleidobrachialis; 4, manubrium of sternum; 5, pectoralis descendens; 6, pectoralis transversus; 7, pectoralis profundus.

The sternocephalicus forms the ventral border of the jugular groove. The dorsal border of the groove is furnished by the brachiocephalicus, described more fully elsewhere (p. 83). The groove is often visible in life, particularly toward the upper part of the neck. It accommodates the external jugular vein (Figure 2–42).

Figure 2–42 Plastination specimen of the ventral part of the neck of a dog. Notice the external jugular vein (1) in the groove formed by the brachiocephalic muscle (2) dorsally, and the sternocephalic muscle (3) ventrally.

The deeper muscles constitute an infrahyoid group closely integrated in arrangement and function. They provide an incomplete cover to the lateral and ventral aspects of the trachea and insert, directly or indirectly, on the hyoid apparatus, which they stabilize and retract toward the thorax during swallowing. The obvious members of the group are the sternothyroideus, sternohyoideus, and omohyoideus; the thyrohyoideus on the lateral aspect of the larynx may be regarded as a detached member. The nerve supply is mainly, although possibly not entirely, from the first and second cervical nerves.

The sternothyroideus and sternohyoideus are very thin ribbonlike muscles that take a common origin from the manubrium of the sternum. The caudal parts of the right and left muscles are not always distinctly divided, and in the middle of the neck they may share a common intermediate tendon from which three or four slips diverge cranially. The sternothyroideus inclines laterally to terminate on the lateral aspect of the thyroid cartilage. The sternohyoideus, not always separable from its fellow, passes beside the midline to insert on the basihyoid.

The omohyoideus, lacking in carnivores, is also thin and straplike. Its absence is compensated by the relative enlargement of the other muscles. In the horse it arises from the subscapular fascia, and in the ruminants from the deep fascia of the neck; thereafter it edges medially to join the lateral margin of the sternohyoideus beside which it inserts. In the horse it provides a floor to the caudal part of the jugular groove, separating the vein from the structures within the visceral space.

Pectoral Girdle

The scapula, or shoulder-blade (Figure 2–45), is a flat bone that lies over the laterally compressed, craniodorsal part of the thorax, where it is held in place by an arrangement (synsarcosis) of muscles without forming a conventional articulation with the trunk. It is the basis of the shoulder region, a term that embraces much more than the immediate neighborhood of the shoulder joint. In ungulates, the scapula is extended dorsally by an unossified portion, the scapular cartilage (Figure 2–45, E/13), which enlarges the area for muscular attachment. The cartilage becomes increasingly calcified and thus more rigid with age.

Figure 2–45 Left scapula of the dog; lateral (A), ventral (B), and medial (C) views. Distal end (D) of left feline scapula. Left equine scapula (E). 1, Cranial angle; 2, spine; 2′, tuber of spine; 3, supraspinous fossa; 4, infraspinous fossa; 5, neck; 6, supraglenoid tubercle; 7, acromion; 7′,7″, hamate and suprahamate processes of acromion; 8, infraglenoid tubercle; 9, caudal angle; 10, facies serrata; 11, coracoid process; 12, glenoid cavity; 13, scapular cartilage.

The bone is roughly triangular, though less so in the dog and cat than in the other domestic species. Its lateral surface is unequally divided by a prominent spine into supraspinous and infraspinous fossae, each occupied by the like-named muscle. The spine extends from the dorsal border almost to the articular angle and may bear a thickening for the insertion of the thoracic part of the trapezius; it is generally palpable through the skin. In all but the horse and pig, it ends in a prominent process (acromion), laterally flattened to form a hamate process in the carnivores (Figure 2–45/7′) and furnished with an additional projection (suprahamate process; Figure 2–45/7″) in the cat. The medial surface of the bone is largely given over to the origin of the subscapularis, which occupies a shallow fossa; a more dorsal roughened area, where the serratus ventralis attaches, extends onto the cartilage in the larger species.

The caudal border is thickened and almost straight. The thinner and sinuous cranial border is notched toward its distal end for the passage of the suprascapular nerve. The dorsal border is also generally straight and extends between cranial and caudal angles; the latter is thickened and more easily identified on palpation. The ventral or articular angle is joined to the body of the bone by a slightly constricted neck. Its caudal part carries a shallow glenoid cavity (Figure 2–45/12) for articulation with the head of the humerus. The cavity, which is somewhat extended in the sagittal direction, faces more or less ventrally. A large muscular process, the supraglenoid tubercle, projects in front of the cavity; it gives origin to the biceps brachii.

The clavicle is reduced to a fibrous intersection in the brachiocephalicus. A nubbin of bone in the dog and a slender rodlet in the cat are embedded in the intersection; their sole importance lies in the risk of misinterpretation when they are seen on radiographs.

Skeleton of the Free Appendage

The humerus (Figure 2–46) forms the skeleton of the arm. It is a long bone that lies obliquely against the ventral part of the thorax, more horizontally in the large species than in the small. It is also relatively shorter and more robust in horses and cattle than in the small ruminants and carnivores. The proximal extremity carries a large articular head (Figure 2–46/2), facing toward the glenoid cavity of the scapula and thus offset to the shaft to which it is joined by a neck. The head is shaped like a segment of a sphere and is considerably larger than the fossa with which it articulates. Two processes, the greater (lateral) and lesser (medial) tubercles, are placed in front and to the side of the articular area. They are separated by the intertubercular groove (Figure 2–46/13) through which the biceps tendon runs. The processes are sometimes more or less equal, as in the horse. More often the lateral one, which forms the basis of the surface feature known as the point of the shoulder, is larger; it is so in the dog. In the horse and in cattle, both tubercles are divided into cranial and caudal parts (Figure 2–46/1′,1″,3′); the intertubercular groove is also molded by an intermediate tubercle in the horse (Figure 2–46/13′). The medial and lateral tubercles give attachment to the muscles that brace and support the shoulder joint, substituting for collateral ligaments.

Figure 2–46 Left humerus of the dog; caudal (A) and cranial (B) views. C, Distal end of right feline humerus; cranial view. Cranial (D) and lateral (E) views of left equine humerus. 1, Greater tubercle; 1′,1″, cranial and caudal parts of greater tubercle; 2, head; 3, lesser tubercle; 3′, cranial part of lesser tubercle; 4, teres (major) tuberosity; 5, deltoid tuberosity; 6, lateral supracondylar crest; 7, olecranon fossa (with supratrochlear foramen in dog); 8, medial epicondyle; 9, condyle; 10, lateral epicondyle; 11, radial fossa; 12, groove for brachialis; 13, intertubercular groove; 13′, intermediate tubercle; 14, supracondylar foramen.

A twisted appearance is imparted to the shaft by a groove (Figure 2–46/12) that spirals over the lateral aspect and carries the brachialis and the radial nerve. Laterally, toward its upper end, the shaft carries the large, easily palpated deltoid tuberosity (Figure 2–46/5), which is joined to the greater tubercle by a prominent ridge. A less prominent, gradually subsiding ridge, the crest of the humerus, continues distally beyond the deltoid tuberosity. The medial aspect of the shaft is marked by the much less salient roughening, the teres (major) tuberosity.

The distal extremity bears an articular condyle (Figure 2–46/9) that is also set at an angle to the axis of the shaft. In large animals it engages with the radius and has the form of a trochlea. In the dog and cat it is divided into a medial area (trochlea) for the ulna and a lateral area (capitulum) for the radius. In all species the caudal part of the groove of the trochlea is continued proximally into a deep (olecranon) fossa (Figure 2–46/7) that receives the anconeal process of the ulna. Two saliences proximal to the articular surface are known as epicondyles. The medial one (Figure 2–46/8) is prominent and forms a right-angled, caudally directed projection that gives origin to the flexor muscles of the carpus and digit. The cranial aspect of the lateral epicondyle (Figure 2–46/10) gives origin to the extensor muscles of the carpus and digit. To the side, each epicondyle gives origin to the corresponding collateral ligament of the elbow joint. In the dog the floor of the olecranon fossa is perforated by a supratrochlear foramen that opens to a much shallower radial fossa on the cranial aspect of the shaft (Figure 2–46/7,11). In the cat alone, the mediodistal part of the humerus is pierced by a supracondylar foramen (Figure 2–46/14) that gives passage to the median nerve and brachial artery.

The skeleton of the forearm is provided by two bones, the radius and the ulna (Figure 2–47). In the standing position they are arranged with the ulna caudal to the radius in the upper part of the forearm but lateral in the lower part. In the primitive condition these bones articulate only at their extremities, leaving an interosseous space between their shafts; rotational movements of the human forearm bones result in turning the hand so that the palm is brought to face forward (supination) or backward (pronation). In most domestic animals the capacity for these movements is reduced or lost, and the two bones are firmly held together by ligaments or by fusion in the prone position. When supination is possible, it consists of rotation of the upper extremity of the radius within the embrace of the ulna while the distal extremity is carried in an arc around the ulna.

Figure 2–47 Left ulna (A) and left radius (B) of the dog. In sequence from the left: cranial view of the ulna, craniolateral and cranial views of the radius and ulna, and caudal view of the radius alone. Cranial (C) and lateral (D) views of fused left radius and ulna of the horse. 1, Olecranon; 2, anconeal process; 3, trochlear notch; 4, 4′, lateral and medial coronoid processes; 5, distal articular facet for radius; 6, lateral styloid process (with facet for the ulnar carpal bone in the dog); 6′, distal end of ulna incorporated within radius; 7, articular facet for ulna; 8, medial styloid process; 9, circumferential facet; 10, radial tuberosity; 11, interosseous space.

Clearly, no movement is possible when the bones are fused, which is the condition prevailing in ungulates and reaching its extreme in the horse, in which only the upper end of the ulna remains distinct (Figure 2–47, D/1). About 45° of supination is allowed to the dog, and somewhat more to the cat. (Rotation at the carpus contributes a substantial extra component to the movement subjectively interpreted as supination.)

The radius is a rather simple rodlike bone, usually much stronger than the ulna in ungulates, but less dominant in carnivores, in particular the cat. The proximal extremity is transversely widened, though tending to a more circular plan in carnivores, in which some supinatory capacity remains. It articulates with the distal articular surface of the humerus and is shaped to match this. A circumferential facet (Figure 2–47, B/9) on the caudal part of the extremity articulates with the ulna and is present even when no supination is allowed. The shaft is craniocaudally compressed and slightly bowed in its length. The distal part of the cranial surface is grooved for the passage of the extensor tendons (Figure 2–47, C), whereas the caudal surface is roughened for muscular attachment. The medial border is subcutaneous and therefore palpable.

The distal extremity of the radius is somewhat expanded. It carries an articular surface that is concave in its cranial part and convex in its caudal part in ungulates; it has a slightly concave ovoid form in carnivores, in which some abduction, adduction, and rotation are allowed to the antebrachiocarpal joint in addition to the major movements of flexion and extension. Medial to the articulation, the radius is prolonged to form a styloid process (Figure 2–47, B/8). The corresponding lateral projection is furnished by the ulna and, in the horse, by the portion of the radius representing the incorporated ulna.

The ulna has an unusual appearance, as its shaft is greatly reduced and its proximal extremity is prolonged beyond the articular surface to form the high olecranon, the point of the elbow. This process, which constitutes a very prominent landmark, gives attachment to the triceps. Distal to this, the cranial margin carries the beaklike anconeal process (Figure 2–47/2), which fits into the olecranon fossa of the humerus, above an articular notch that engages with the humeral trochlea; yet farther from the extremity, there is a facet for the circumferential articular area of the radius. In the dog the shaft, though slender, runs the full length of the radius from which it is separated by an interosseous space that is bridged by membrane in life. The distal extremity carries a small articular facet for the radius and beyond this is continued as the lateral styloid process (Figure 2–47/6), which makes contact with the ulnar carpal bone.

Reduction of the ulna is greatest in the horse, in which the shaft tapers to end at midforearm level (Figure 2–47, D). The distal part became incorporated within the radius in fetal life (Figure 2–47/6′). The ruminants and pig show intermediate conditions. Of course the fusion of the ulna with the radius prohibits the movements of supination and pronation in the domestic mammals other than the dog and cat.

The short carpal bones articulate in complex fashion. The plan of the primitive carpal skeleton is uncertain, but in domestic species the bones are clearly arranged in two rows (Figure 2–48). The proximal row comprises (in mediolateral sequence) radial, intermediate, ulnar, and accessory bones; the last appears as an appendage projecting behind the carpus and is a prominent landmark in the live animal. The radial and intermediate carpals fuse in the dog and cat. The elements of the distal row are numbered from one to five (again in mediolateral sequence), although the fifth never appears as a separate bone but is either suppressed or fused with the fourth. The first is also often lacking while the second and third fuse in ruminants. The diagrams illustrate the carpal formulae in different species. Apart from the accessory carpal bone, which is probably a sesamoid by origin, a small sesamoid bone is embedded in the medial tissues of the joint of the dog. Intrinsically unimportant, it can confuse radiographic interpretation by wrongly suggesting a “chip” fracture.

Figure 2–48 The bones of the carpal skeleton in the carnivores (Car), horse (eq), cattle (bo), and pig (su), schematic. Roman numerals identify the metacarpal bones; Arabic numerals, the distal carpal bones. R, Radius; U, ulna; a, accessory carpal bone; i, intermediate carpal bone; r, radial carpal bone; u, ulnar carpal bone.

Viewed as a whole, the carpus is convex from side to side on its cranial aspect and flat and very irregular caudally, although in life these irregularities are smoothed by thick ligaments. Most movement occurs at the antebrachiocarpal level, some at the intercarpal level, and virtually none at the carpometacarpal level or between neighboring bones in a row. The combined proximal articular surface is the reciprocal of that of the radius (see earlier) and in carnivores has a convex ovoid form.

The primitive pattern for the skeleton of the mammalian manus exhibits five more or less equal rays, each consisting of a metacarpal bone and proximal, middle, and distal phalanges in line (Figure 2–49, A). This pattern has been modified in all domestic species, each of which (not excepting the pig) is to some degree specialized for fast running. Cursorial specialization involves raising the manus (and pes) from the primitive “flatfooted” (plantigrade) posture demonstrated by bears (Figure 2–50). An intermediate stage, the digitigrade posture, has been attained by dogs, which support themselves by the digits only; it culminates in the unguligrade posture attained by ruminants, pigs, and horses, in which only the tips of the digits, protected by hooves (ungulae), give support. The process results in the abaxial digits first losing permanent contact with the ground; a compensating development of the remaining digits enables them to carry an increased proportion of the weight. The process has not progressed very far in the dog and cat, in which only the most medial (first) digit has lost contact and is retained as a nonfunctional dewclaw (Figure 2–51). The four functional digits are broadly equal, with the axis of the manus passing between the third and fourth digits (a paraxonic position). Pigs have entirely lost the first digit; the second and fifth digits are very much reduced, although each retains a complete skeleton. In ruminants the process has gone further, and although elements of four digits are present, those of the abaxial pair are vestigial; the metacarpal bones of the functional third and fourth digits are fused in a single bone that retains evidence of its composite origin (Figure 2–49, C).

Figure 2–49 Right manus (human hand; A) of horse (B) and ruminant (C), palmar views. The Roman numerals number the rays. 1, Radius; 2, ulna; 3, metacarpal; 4, 5, 6, proximal, middle, and distal phalanges; 7, carpal bones; 8, rudimentary metacarpal V; 9, accessory carpal bone; 10, rudimentary metacarpals II and IV (medial and lateral splint bones); 11, axis in line with ray III (mesaxonic), in C paraxonic.

Figure 2–50 Hindlimbs of bear, dog, and horse (from left to right) illustrating plantigrade, digitigrade, and unguligrade postures.

Figure 2–51 Skeleton of the right manus of the dog, lateral (A) and dorsal (B) views. The Roman numerals identify the metacarpal bones. 1, Radius; 2, ulna; 3, accessory carpal; 4, ulnar carpal; 5, radial carpal (intermedioradial in the dog); 5′, intermediate carpal; 6, 7, first and fourth of the distal row of carpal bones; 8, sesamoid bone; 9, proximal sesamoid bones; 9′, ridged articular surface of equine metacarpus III, articulates with proximal sesamoid bones (not shown); 10, dorsal sesamoid bone; 11, 12, 13, proximal, middle, and distal phalanges; 13′, claw; 14, axis of manus.

In the horse (Figure 2–49, B), only the third ray survives in functional form and its axis coincides with that of the limb; the manus is said to be mesaxonic. Remnants of the second and fourth metacarpal bones survive as the splint bones that flank the third metacarpal or cannon bone; they end in nodules, but the assumption that these incorporate greatly reduced elements of all three phalanges of the lost digits is unfounded.

The differences in the metacarpal and digital skeleton are very striking as a consequence of these changes, and the short description that follows is amplified in later chapters by details of a species-specific nature.

As the number of metacarpal bones diminishes, the relative stoutness of the surviving members of the series increases. The single (third) metacarpal bone of the horse therefore has a particularly strong shaft, whereas the individual metacarpal bones of the dog are relatively much weaker. The dog’s bones are also shaped by their mutual contacts; the third and fourth bones are square in section, and the flanking second and fifth bones are triangular. Taken as a whole, the metacarpal skeleton of all species is somewhat compressed in the dorsopalmar direction. Each bone has a proximal extremity (base), a shaft, and a distal extremity (caput). The base has a flattish articular surface for the distal row of carpal bones and may, according to its position in the metacarpal series, have medial and lateral facets where it makes contact with neighbors. The distal extremity articulates with the proximal phalanx by a hemicylindrical surface with a central ridge. Various roughenings for ligamentous attachment are present at both extremities.

The proximal phalanx is a short cylindrical bone with a proximal extremity adapted to the caput of the metacarpal bone and a distal articulation in the form of a shallow trochlea. Again, the bone may be shaped by its position in the digital series.

The middle phalanx is shorter than, but basically very similar to, the first phalanx. The distal phalanx corresponds to the form of the hoof or claw in which it is wholly (hoof) or partly (claw) contained. The digital skeleton is completed by paired proximal sesamoid bones at the palmar aspect of the metacarpophalangeal joint and by a distal sesamoid bone (cartilage in the dog) at the palmar aspect of the distal interphalangeal joint. In the dog small sesamoids also exist within the extensor tendons over the dorsal aspect of the metacarpophalangeal joints.

Girdle Muscles

The girdle muscles join the forelimb to the trunk, forming a connection known as a synsarcosis that substitutes for a conventional joint. When the animal is standing, some of the muscles of the synsarcosis (the serratus ventralis and pectoralis profundus) sling the body between the forelimbs to which they transmit the weight of the head, neck, and cranial part of the trunk (Figure 2–54). These and other girdle muscles can also stabilize the scapula against external forces, preventing its displacement or rotation. A good example of this role is supplied by a cat pouncing on a mouse or plaything with forelimbs rigidly braced against the trunk. During progression the same muscles resolve into antagonistic groups that control the swing of the limb; one group advances (protracts) the limb, the other retracts it. For these actions to be understood, it is necessary to appreciate that the scapula may be moved against the chest wall in two different ways. In one, the bone is rotated about a transverse axis located toward its upper end. The position of this axis, which is of course imaginary, is fixed by the balance of opposing muscles, chiefly the rhomboideus and serratus ventralis, which both attach on the dorsal part of the scapula. In the other movement, the whole bone is shifted on the thoracic wall. It is slid downward and forward as the limb is advanced and upward and backward in recovery during retraction. This movement of the scapula, which adds usefully to the length of the stride, is permitted by the looseness of the connective tissue that intervenes between the limb and the trunk where there exists a potential space, the axilla, corresponding to the human armpit. The axilla also gives passage to the nerves and vessels entering the limb from the trunk, and it contains the axillary lymph nodes.

Figure 2–54 Muscular suspension of the thorax between the forelimbs (dog). 1, Scapula; 2, humerus; 3, radius and ulna; 4, sternum; 5, pectoralis profundus (ascendens); 6, serratus ventralis; 7, trapezius; 8, rhomboideus.

For the purpose of description, the girdle muscles can be considered in two layers.

The Superficial Layer

This consists of a cranial group supplied mostly by the accessory nerve, the latissimus dorsi more caudally, and the two superficial pectoral muscles ventrally. The cranial group comprises the trapezius, omotransversarius, and brachiocephalicus.

The trapezius (Figure 2–55/5,5′) is thin. It takes origin from the middorsal raphe and supraspinous ligament, extending from about the level of the second cervical to that of the ninth thoracic vertebra, and converges to insert on the spine of the scapula. It consists of two fleshy parts, cervical and thoracic, usually separated by an intermediate aponeurosis. The fibers of the cervical part run caudoventrally to attach along the greater part of the length of the scapular spine; those of the thoracic part run cranioventrally to a more confined insertion on the tuberous thickening of the spine. The trapezius may raise the scapula against the trunk and swing the ventral angle of the bone cranially, thus advancing the limb.

Figure 2–55 Superficial muscles of the shoulder and arm. 1, Sternocephalicus; 2, 2′, brachiocephalicus: cleidocervicalis and cleidobrachialis; 3, omotransversarius; 4, superficial cervical lymph node; 5, 5′, cervical and thoracic parts of trapezius; 6, deltoideus; 7, latissimus dorsi; 8, 8′, long and lateral heads of triceps; 9, pectoralis profundus (ascendens); 10, accessory axillary lymph node.

The omotransversarius (Figure 2–55/3) is a narrow muscle that extends between the transverse processes of the atlas (and possibly also the succeeding vertebrae) and the acromion and adjacent part of the scapula. It assists in advancement of the limb.

The brachiocephalicus (Figure 2–55/2,2′) is more complex, being formed by the union of two elements that are separated by the clavicle in less specialized mammals. In these the caudal part (cleidobrachialis) passes between the clavicle and the humerus and is a component of the deltoideus muscle. The cranial part passes cranially from the clavicle to several attachments in the head and neck. These attachments vary among species and hence a rather bewildering array of names for particular units exists: cleidooccipitalis, cleidomastoideus, and so forth. In domestic species the two parts join in tandem, and the clavicle is generally reduced to a fibrous intersection in the combined muscle at the level of the shoulder joint, although vestigial ossifications are present in the dog and cat. Brachiocephalicus is a most appropriate name for the whole complex since it does not specify precise attachments. The brachiocephalicus advances the limb, possibly also extending the shoulder joint, when the cranial attachment is fixed and the limb is free to move; in contrast, when the limb is fixed and the head is free, it draws the head and neck ventrally when acting bilaterally and toward the side when acting unilaterally.

The muscles supplied by the accessory nerve split from a single primordium in the embryo. However, the caudal part of the brachiocephalicus of deltoid origin retains the appropriate innervation by the axillary nerve.

The latissimus dorsi (Figure 2–55/7) has a very broad origin from the thoracolumbar fascia and converges to an insertion on the teres tuberosity of the humerus. The most cranial fibers, which are also the most vertical, cover the caudal angle of the scapula and strap it against the chest. The muscle retracts the free limb and may also flex the shoulder joint. On the other hand, when the limb is advanced and the foot firmly planted on the ground, the latissimus may draw the trunk forward. It may be regarded as antagonist to the brachiocephalicus. It is supplied by a local branch (thoracodorsal nerve) of the brachial plexus.

Two superficial pectoral muscles (Figure 2–41/5,6) arise, one behind the other, from the cranial part of the sternum. The cranial muscle (pectoralis descendens) terminates on the crest of the humerus, distal to the deltoid tuberosity. The caudal muscle (pectoralis transversus) descends over the medial aspect of the arm and in the larger species continues distally over the elbow joint, covering the median artery and nerve, to insert into the medial fascia of the forearm. Both muscles adduct the limb, which is an action that may be understood to embrace the sideways shift of the trunk toward a previously abducted limb. It seems probable that they may also assist protraction or retraction, depending on the initial position of the limb relative to the trunk. They are supplied by local branches (cranial pectoral nerves) from the brachial plexus.

Intrinsic Muscles of the Forelimb

The intrinsic muscles are conveniently grouped by their common location, actions, and innervations.

Muscles Acting Primarily on the Shoulder Joint

The muscles acting on the shoulder joint are arranged in lateral, medial, and caudal groups.

The lateral group comprises the supraspinatus and infraspinatus, which arise from and fill the corresponding fossae of the scapula. The supraspinatus (Figure 2–56/3) terminates on the summits of both tubercles of the humerus. The infraspinatus inserts by a tendon that splits into a shorter deep part, which attaches to the summit, and a longer superficial part, which attaches to the lateral face of the (caudal part of the) greater tubercle; a bursa between the bone and the longer tendon may be the seat of a painful inflammation. Both muscles brace the joint laterally. The supraspinatus tendon passes cranial to the axis of rotation, and it may therefore also extend the shoulder. It is sometimes asserted that the infraspinatus tendon passes cranial or caudal to the axis of rotation depending on the actual position of the joint and may then further extend the already extended joint or further flex the already flexed joint; clearly, it is unlikely to be very effective in either role. Both muscles are supplied by the suprascapular nerve from the brachial plexus.

Figure 2–56 Intrinsic muscles of the left shoulder and arm of the dog, lateral (A) and medial (B) views. 1, Rhomboideus; 2, teres major; 3, supraspinatus; 4, 4′, scapular and acromial parts of deltoideus; 5, latissimus dorsi; 6, 6′,6″, long, lateral, and medial heads of triceps; 7, brachiocephalicus; 8, brachialis; 9, subscapularis; 10, coracobrachialis; 11, tensor fasciae antebrachii; 12, biceps.

The medial group comprises the subscapularis and coracobrachialis. The subscapularis (Figure 2–56/9) arises over much of the deep surface of the scapula and inserts on the medial tubercle of the humerus, distal to the axis of the shoulder joint. It braces the medial aspect of the joint. It is also a potential adductor of the arm and, like the infraspinatus, has an equivocal relationship to flexion and extension of the shoulder. It is supplied by the subscapular nerve from the brachial plexus. The coracobrachialis (Figure 2–56/10) extends between the medial aspect of the supraglenoid tubercle and the proximal part of the shaft of the humerus. Too small to be of real significance, it is a fixator of the shoulder with the same equivocal relationship to shoulder flexion and extension. It is supplied by the proximal branch of the musculocutaneous nerve from the brachial plexus.

The caudal or flexor group comprises the deltoideus, teres major, and teres minor. The deltoideus has one head of origin in the horse and two in species possessing an acromion (Figure 2–56/4,4′). The constant head arises from the caudal border and spine of the scapula; the inconstant second head arises from the acromion. Both insert on the deltoid tuberosity of the humerus. The teres major (Figure 2–56/2) arises from the dorsal part of the caudal margin of the scapula and terminates on the teres tuberosity, midway down the humerus. The relatively insignificant teres minor lies over the caudolateral aspect of the joint between the deltoideus and infraspinatus. These three muscles are clearly primarily flexor; the deltoideus may also be an abductor and an outward rotator of the arm. The group is supplied by the axillary nerve from the brachial plexus.

In contrast to the well-defined group of flexors, it seems that no muscles are clearly established as primarily extensors of the shoulder. The potential candidates, brachiocephalicus, biceps brachii, supraspinatus, and pectoralis ascendens, have other, apparently more important roles.

Pronator and Supinator Muscles of the Forearm

Generalized mammals possess muscles that have supination or pronation as a prime function, but these muscles tend to become vestigial or to disappear when the capacity for the movements is reduced or lost. Among domestic species significant movement is possible only in the dog and cat in which there are two supinator muscles and two pronators. The brachioradialis or long supinator is a thin fleshy ribbon that extends from the lateral epicondyle of the humerus to the distal medial part of the forearm within the superficial fascia. It is quite prominent in the cat but is slight, often lost, in the dog. The short supinator muscle is more consistently developed. It is a small fusiform muscle, placed deep to the extensor muscles and passing obliquely over the flexor aspect of the elbow from the lateral humeral epicondyle to the upper quarter of the medial border of the radius. The supinator muscles are supplied by the radial nerve.

The pronator teres (Figure 2–57/12) arises from the medial epicondyle of the humerus and converges on the insertion of the supinator on the radius. It is functional only in the dog and cat. The pronator quadratus is found only in carnivores. It passes from the shaft of the ulna to that of the radius, bridging the medial aspect of the interosseous space of the forearm. The pronator muscles are supplied by the median nerve.

Figure 2–57 Muscles of the left forearm of the dog, lateral (A) and medial (B) views. 1, Extensor carpi radialis; 2, common digital extensor; 3, lateral digital extensor; 4, ulnaris lateralis; 5, flexor carpi ulnaris; 6, extensor carpi obliquus; 7, extensor retinaculum; 8, carpal pad; 9, biceps; 10, superficial digital flexor; 11, flexor carpi radialis; 12, pronator teres; 13, radius; 14, deep digital flexor; 15, flexor retinaculum.

The rotation from the neutral position that may be produced by these muscles is most free when the elbow is flexed. The movements are limited to about 40° of pronation and about 45° of supination in the dog, although the cat has a somewhat larger range.

Pelvic Girdle

The pelvic girdle has been described with the trunk (p. 43) for the reason previously given.

Skeleton of the Free Appendage

The femur (os femoris; Figure 2–58), the skeleton of the thigh, is the strongest of the long bones. The proximal end curves medially so that the proximal articular surface, the head, is offset to the long axis of the shaft. The femoral head is hemispherical and is joined to the shaft by a neck, which is best defined in the smaller species. The articular surface is interrupted by a nonarticular area (fovea) to which the intracapsular ligament(s) attach(es); the fovea is round and central in the dog, and wedge-shaped and extended to the medial periphery in the horse. A large process, the greater trochanter (Figure 2–58/3), is placed lateral to the head; it rises level with the head in small animals but projects high above it in larger species (Figure 2-58/3′,3″); it gives attachment to the bulk of the gluteal muscles, providing these extensors of the hip with a long lever arm. A plate of bone between the trochanter and the femoral neck helps bound the trochanteric fossa (Figure 2–58/5), an excavation that is open caudally, and the site of insertion of the small rotator muscles of the hip.

Figure 2–58 Left femur of the dog, cranial (A), caudal (B), and lateral (C) views. Cranial (D) and lateral (E) views of left equine femur. 1, Head; 1′, fovea; 2, neck; 3, greater trochanter; 3′,3″, cranial and caudal parts of greater trochanter; 4, lesser trochanter; 4′, third trochanter; 5, trochanteric fossa; 6, trochlea; 6′, enlarged proximal end of medial trochlear ridge; 7, supracondylar tuberosities; 7′, supracondylar fossa; 8, 8′, lateral and medial condyles; 9, intercondylar fossa; 10, patella; 11, sesamoid bones (in gastrocnemius); 12, extensor fossa; 13, fossa for popliteus.

The caudal aspect of the shaft is flattened, but the other aspects combine in a continuous smooth surface. The borders between the flat and rounded areas are emphasized by rough lines indicating muscular attachment. Two processes mark the proximal half of the shaft. A low and rough lesser trochanter (Figure 2–58/4) projects from the medial border and gives insertion to the iliopsoas muscle. An inconspicuous ridge at the base of the greater trochanter is known as the third trochanter (trochanter tertius; Figure 2–58/4′). It is salient only in the horse and gives attachment to the gluteus superficialis. In the large animals the caudodistal part of the shaft exhibits a deep supracondylar fossa that increases the area of origin of the superficial digital flexor (Figure 2–58/7′). The same function is fulfilled by tuberosities in the dog.

The distal extremity articulates with the tibia and the patella. The articulation with the tibia is accomplished by two condyles directed caudodistally and separated by a deep intercondylar fossa. The abaxial surfaces of the condyles are roughened and give attachment to the collateral ligaments of the stifle. The lateral condyle also carries two depressions close to the articular margin: the cranial one, the extensor fossa (Figure 2–58/12), gives origin to the long digital extensor and peroneus tertius muscles; the caudal one (Figure 2–58/13) gives origin to the popliteus. In the dog and cat the caudal aspect of each condyle is surmounted by a small flat facet for articulation with one of the small sesamoid bones (Figure 2–58/11; formerly fabellae) in the origin of the gastrocnemius (see Figure 17–3). A cranial trochlea (Figure 2–58/6) articulates with the patella and extends proximally on the cranial surface. The bounding ridges are low and more or less equal in size in the dog and relatively larger and disparate in the horse and in cattle, in which the stouter medial ridge ends in a proximal enlargement (Figure 2–58/6′).

The patella, the kneecap, is a sesamoid developed within the insertion of the quadriceps femoris, the main extensor of the stifle. It is ovoid in the dog but prismatic in the horse and in cattle. It is extended medially and laterally by parapatellar cartilages in the fresh state.

The skeleton of the leg consists of the tibia and fibula (Figure 2–59), which, unlike the analogous elements of the forelimb, run side by side without any tendency to cross. The medial bone, the tibia, is always by far the larger of the two. The fibula is excluded from articulation with the femur and has only restricted contact with the hock skeleton.

Figure 2–59 Left tibia and fibula of the dog, lateral (A), cranial (B), and caudal (C) views. Cranial (D) and lateral (E) views of left equine tibia and fibula. 1, Tibial tuberosity; 2, 2′, lateral and medial condyles; 3, extensor groove; 4, intercondylar eminence; 5, fibula; 6, 6′, medial and lateral malleoli; 6″, lateral malleolus in the horse (representing distal end of fibula); 7, cochlea.

The expanded proximal extremity of the tibia presents two condyles divided by a caudal popliteal notch that accommodates the like-named muscle. Each condyle has a gently undulating articular surface facing the corresponding condyle of the femur; a narrow intermediate nonarticular area carries a central eminence (Figure 2–59/4) onto which the articular surfaces slope. A depression of the eminence and less defined areas cranial and caudal to it indicate ligamentous attachments. The very robust tibial tuberosity (Figure 2–59/1) projecting from the cranial aspect of this extremity is a prominent landmark in life; it is continued by a gradually subsiding crest. A groove (Figure 2–59/3) lodging the tendons of certain muscles of the leg (crus) separates the tuberosity from the cranial aspect of the lateral condyle. Caudal to this, the edge of the condyle carries a small facet for articulation with the fibula, although in some species the joint space is obliterated by fusion.

The proximal part of the tibial shaft is three-sided, but more distally the bone is craniocaudally compressed; the change is brought about by the smooth surface that faces craniolaterally in its proximal part but then twists to face directly forward. The entire medial surface (border distally) is subcutaneous and flat. The caudal surface is ridged for muscular attachment.

The distal extremity carries an articular area known as the cochlea (Figure 2–59/7), which is shaped to receive the trochlea of the talus. The central ridge and the flanking grooves of the cochlea have a craniolateral deflection, although the angle varies among species. A bony salience, the medial malleolus (Figure 2–59/6), is present to the medial side of the cochlea. A similar lateral swelling is found only in the horse and represents the assimilated distal part of the fibula (Figure 2–59/6″). In other species the corresponding feature (lateral malleolus) is provided by the fibula.

In carnivores and the pig the fibula is reduced in robustness but not in length. It is separated from the tibia by an interosseous space that runs the whole length of the leg in the pig but is limited to the proximal half in the dog. The shaft of the fibula regresses in ruminants: the proximal extremity persists as a tear-shaped process fused to the lateral condyle of the tibia; the distal extremity is isolated as a small compact malleolar bone that forms an interlocking joint with the tibia, completing the articular surface for the talus. The flattened proximal head of the fibula of the horse is closely applied to the tibia, and the slender shaft that leads from it converges on the tibia but fades toward the middle of the leg.

The tarsal bones are arranged in three tiers. The proximal tier consists of two relatively large bones: the talus medially and the calcaneus laterally. The middle tier comprises only a single central tarsal bone, but the distal tier comprises up to four bones, which are numbered in mediolateral sequence. The lateral fourth tarsal bone is constantly present and, being much deeper than the others, intrudes into the middle tier (Figure 2–60).

Figure 2–60 The bones of the tarsal skeleton in the carnivores (Car), horse (eq), cattle (bo), and pig (su), schematic. Roman numerals identify the metatarsal bones, Arabic numerals the distal tarsal bones. Tib., Tibia; F, fibula; T, talus; C, calcaneus; c, central tarsal bone.

The talus (Figure 2–61) has a proximal trochlear surface shaped to fit the tibia. The distal surface, which articulates with the central bone, is flattened in the horse and more rounded in other species. The calcaneus lies mainly lateral to the talus but extends a shelflike process that overlaps the talus on its plantar surface; the process (sustentaculum tali; Figure 2–61/3′) supports the deep digital flexor tendon. The larger part of the bone projects proximally behind the tibia as a free lever arm to which the common calcanean tendon attaches. It ends in a thickening that is the basis of the point of the hock (Figure 2–61/3″) and corresponds to the human heel. The distal extremity of the calcaneus rests on the fourth tarsal bone (Figure 2–61/6). The central tarsal bone is interposed between the talus proximally and the first, second, and third tarsal bones distally; its proximal surface conforms to the talus and is concave in most animals but flat in the horse. Its distal articular surface is flattened. The central and fourth tarsal bones fuse in ruminants.

Figure 2–61 Skeleton of right pes of the dog, lateral (A) and dorsal (B) views. Dorsal (C) view of left equine tarsus. Roman numerals identify the metatarsal bones. 1, Tibia; 2, fibula; 2′, lateral malleolus; 3, calcaneus; 3′, sustentaculum tali; 3″, calcanean tuber (point of hock); 4, talus; 5, central tarsal; 6, fourth tarsal; 7, first, second, and third tarsal bones in distal row; 7′, third tarsal in the horse; 8, proximal sesamoid bones; 9, dorsal sesamoid bones; 10, 11, 12, proximal, middle, and distal phalanges; 12′, claw.

The distal tarsal bones are not always separate: the first and second are fused in the horse, and the second and third are fused in ruminants. Individually irregular, these bones together form a more or less flattened disk interposed between the central tarsal and the metatarsal bones. The cuboidal fourth tarsal bone is interposed between the calcaneus and the lateral metatarsal bones; in some species it also gives support to the talus.

The remaining bones of the hindlimb closely resemble those of the forelimb. The metatarsal bones are longer (by about 20%) than the metacarpals and are more rounded in cross section. The first metatarsal is rudimentary in the dog, in which only a few breeds consistently possess a dewclaw in the hindlimb.

The Intrinsic Muscles of the Hindlimb

Muscles Acting Primarily on the Hip Joint

The muscles acting at the hip are arranged in gluteal, medial, deep, and caudal (hamstring) groups; it is a classification based primarily on topography.

The gluteal group comprises superficial, middle, and deep gluteal muscles and the tensor fasciae latae. The gluteus superficialis varies greatly. In the dog it is a relatively narrow muscle that covers the caudal part of the gluteus medius, extending from the gluteal and caudal fascia to the third trochanter of the femur (Figure 2–64/4). In ungulates a part becomes incorporated within the biceps femoris, and sometimes also the semitendinosus, supplying these with vertebral heads of origin. It is an extensor of the hip and therefore a retractor of the limb. It is supplied by the caudal gluteal nerve.

Figure 2–64 Muscles of the canine hindquarter and thigh, lateral (A) and medial (B) views. 1, Sartorius; 2, tensor fasciae latae; 3, gluteus medius; 4, gluteus superficialis; 5, biceps; 6, semimembranosus; 7, semitendinosus; 8, pelvic symphysis; 9, internal obturator; 10, levator ani; 11, rectus abdominis; 12, quadriceps; 13, pectineus; 14, adductor; 15, gracilis.

The gluteus medius (Figure 2–64/3) is by far the largest of the group. It arises from the outer surface of the ilium and the gluteal fascia and inserts on the greater trochanter. It is an exceptionally powerful extensor of the hip with some abduction potential. A deeper subdivision is known as gluteus accessorius. Neither it nor the small, more caudal piriformis need be considered separately; their actions are similar to those of the main mass. The muscle is principally supplied by the cranial gluteal nerve.

The much smaller gluteus profundus is completely covered by the gluteus medius. It arises from the ischial spine and adjacent region of the os coxae and inserts on the cranial part of the greater trochanter. It may also extend the hip, but because most fibers run more or less transversely, it is more advantageously placed to abduct the limb. It is also supplied by the cranial gluteal nerve.

The tensor fasciae latae (Figure 2–64/2) is the most cranial muscle of the group. It arises from the coxal tuber (or equivalent) and from the adjacent part of the ilium and extends down the cranial border of the thigh before inserting into the heavy lateral femoral fascia, which serves as its insertion tendon and provides it with attachment to the patella and other structures of the stifle region. Supplied by the cranial gluteal nerve, it is primarily a flexor of the hip. In the horse its most caudal part extends toward and fuses with a cranial slip of the gluteus superficialis.

The medial group is principally employed to adduct the hindlimb, adduction is, of course, a term that also embraces the prevention of unwanted abduction. Most muscles of this group are supplied by the obturator nerve and these—gracilis, pectineus, adductor, and external obturator—are sometimes specifically termed the adductors. The sartorius has a rather different origin and relationship.

The gracilis, a broad but thin muscle, takes an aponeurotic origin from the symphysial region of the pelvis (Figure 2–64/15). Its insertion, also aponeurotic, merges with the crural fascia through which it finds attachment to the tibial crest and other medial structures of the stifle region.

The pectineus is a small fusiform muscle, which in the dog forms a prominent surface feature of the proximal part of the thigh (Figure 2–64/13). It arises from the cranial branch of the pubis and from the prepubic tendon and inserts on the proximal part of the medial “rough line” of the femur. In the larger species, but not in the dog, a considerable part of the tendon of origin decussates with its fellow within the prepubic tendon.

The adductor is often divided into several individually named parts, but these distinctions are unnecessary. The muscle arises over an extensive area of the ventral aspect of the pelvic floor and inserts along the distal two thirds of the medial “rough line” of the femur and to the fascia and ligaments of the medial aspect of the stifle (Figure 2–64/14).

The obturator externus is conveniently included here, although it has obvious affinities with the following deep group. It arises from the ventral surface of the pelvic floor, over and around the obturator foramen, and inserts within the ventral part of the trochanteric fossa. In addition to being an adductor, it is potentially an outward rotator of the thigh.

The sartorius is set apart from the other medial muscles by its innervation from the saphenous branch of the femoral nerve. It is superficial and follows the craniomedial aspect of the thigh; in the dog it consists of two parallel bellies, one of which forms the cranial contour of the thigh (Figure 2–64/1). Except in the horse (in which it arises from the iliac fascia on the abdominal roof), it arises from the iliac crest and its insertion is to the medial structures of the stifle region. Flexion of the hip is probably its main action, but it has some capacity for adduction of the thigh and extension of the stifle. The superficial space between the caudal margin of the sartorius and the pectineus is often designated the femoral canal.

The deep muscles of the hip form a rather heterogeneous community of small and essentially trivial muscles: the obturator internus, gemelli, quadratus femoris, and articularis coxae. Most are supplied by the sciatic nerve.

The obturator internus (Figure 2–64/9) is a thin muscle that arises from the dorsal surface of the hip bone in the vicinity of the obturator foramen; in carnivores and in the horse its tendon leaves the pelvis by passing over the ischium, caudal to the acetabulum, to end in the trochanteric fossa. In other species the tendon passes through the obturator foramen; in this arrangement, the muscle may have its origin as a detachment from the external obturator. The muscle is an external rotator of the thigh.

The gemelli are two small “twin” bundles that pass from the ischial spine to the trochanteric fossa. They are also external rotators.

The quadratus femoris passes from the ventral aspect of the ischium to end on the femoral shaft close to the trochanteric fossa. It is described as an extensor but can be of no significance in this role.

The articularis coxae lies on the capsule over the cranial aspect of the hip and protects this from being nipped between the femoral and acetabular surfaces.

The muscles of the caudal (hamstring) group—biceps femoris, semitendinosus, and semimembranosus—flesh the caudal part of the thigh. They extend from the ischial tuber and adjacent part of the sacrotuberous ligament to a broad insertion both proximal and distal to the joint space of the stifle; certain components continue within the common calcanean tendon to the calcaneus. In ungulates, one (or more) muscle is also extended proximally through the acquisition of an origin (vertebral head) from the sacrocaudal vertebrae. The vertebral heads are best developed in the horse and account for the full, rounded contour of the rump of this animal, which contrasts with the more angular appearance in the ox or dog. At least part of the vertebral extension is due to assimilation of a superficial gluteal component. The term gluteobiceps may be encountered for the combination.

The biceps femoris is most lateral (Figure 2–64/5). In the horse and in ruminants, but not in the dog, it has both vertebral and pelvic heads. In the lower part of the thigh the united muscle divides into insertions that attach, by way of the femoral and crural fascia, to the patella and ligaments of the stifle joint both proximal and distal to the joint space; an additional insertion to the point of the hock is achieved through a contribution (tarsal tendon) to the common calcanean tendon.

The semitendinosus (Figure 2–64/7) forms the caudal contour of the thigh. It has a vertebral head only in the horse and pig. The insertion is to the medial aspect of the proximal extremity of the tibia and to the calcaneus. The insertions of the biceps and semitendinosus, one to each side of the depression (popliteal fossa) behind the stifle, can be palpated in life—they are the “strings of the ham” that give the group its name.

The semimembranosus (Figure 2–64/6) is most medial and has a vertebral head only in the horse. The insertion is divided between a cranial part attaching to the medial femoral condyle and a caudal part attaching to the medial tibial condyle.

In the dog a ribbon-like abductor cruris caudalis lies on the deep face of the biceps and is probably derived from it. It has no great functional significance.

The vertebral heads of these muscles are generally supplied by the caudal gluteal nerve, and the pelvic heads are generally supplied by the sciatic nerve (or its tibial division).

Certain functions of these muscles are difficult to analyze, but their main role is undoubtedly the forceful extension of the hip joint that thrusts the trunk forward. In addition, the biceps has an abductor potential, and the semimembranosus an adductor potential, at the hip.

When consideration is given to muscle action on the stifle, it is probably more useful to divide the muscles into a cranial division inserting proximal to the joint axis and a caudal division inserting distal to this axis rather than to consider the named units. The cranial division extends the stifle when the foot is planted on the ground. The caudal division has the same action when the foot is fixed but flexes the joint when the foot is free to move. The parts of the biceps and semitendinosus that insert on the calcaneus can obviously extend the hock. It is clear that not all these effects can be accomplished simultaneously; apart from the potential antagonism of the cranial and caudal divisions at the stifle, it is unlikely that an animal would wish to flex the stifle while extending the hock. Indeed, in the horse in particular, this combination of actions is precluded by the reciprocal mechanism (p. 637). Different parts of these muscles must therefore be used at different times and in different combinations.

Muscles Acting Primarily on the Stifle Joint

There are extensor and flexor groups. The quadriceps femoris, the principal extensor of the stifle, forms the mass of muscle cranial to the femur (see Figure 17–2/9). It consists of four parts, separate at their origins but joined distally. One, the rectus femoris, arises from the shaft of the ilium immediately cranial to the acetabulum. The others, vastus medialis, intermedius, and lateralis, arise from the medial, cranial, and lateral aspects of the femoral shaft. The common insertion appears to be on the patella but is actually on the tibial tuberosity because the muscle is continued distal to the patella by the patellar ligament(s). The rectus femoris has the potential secondary action of flexion of the hip, although it is ill-placed for this purpose. The quadriceps is supplied by the femoral nerve.

The small popliteus muscle lies directly over the caudal aspect of the joint. It takes a tendinous and confined origin from the lateral condyle of the femur and fans out to a broad fleshy insertion on the proximal third of the caudal surface of the tibia (Figure 2–65/15). Its tendon of origin contains a sesamoid in the dog and cat. In addition to being a flexor of the stifle, the popliteus rotates the distal part of the limb. It is supplied by the tibial nerve.

Figure 2–65 Muscles of the left canine leg, lateral (A) and medial (B) views. 1, Biceps; 2, semitendinosus; 3, peroneal nerve; 4, gastrocnemius; 5, tibialis cranialis; 6, peroneus longus; 7, lateral deep digital flexor, 7′, tendon of the smaller medial deep digital flexor; 8, superficial digital flexor; 9, long digital extensor; 10, peroneus brevis; 11, extensor brevis; 12, tendon of lateral digital extensor; 13, interossei; 14, tibia; 15, popliteus.