The Skeletal and Muscular Systems

Scapula

The scapula (Figs. 2-3 and 2-4), a flat, roughly triangular bone, possesses two surfaces, three borders, and three angles. The ventral angle is the distal or articular end that forms the glenoid cavity, and the constricted part that unites with the expanded blade is referred to as the neck.

The lateral surface (Fig. 2-3) of the scapula is divided into two nearly equal fossae by a shelf of bone, the spine of the scapula. The spine is the most prominent feature of the bone. It begins at the dorsal border as a thick, low ridge and becomes thinner and wider toward the neck. In all breeds the free border is slightly thickened, and in some it is everted caudally. The distal end is a truncated process, the acromion, where part of the deltoideus muscle arises. On a continuation of the spine proximally, the omotransversarius attaches. The remaining part of the spine provides a place for insertion of the trapezius and for origin of that part of the deltoideus that does not arise from the acromion.

The supraspinous fossa is the entire lateral surface cranial to the spine of the scapula. The supraspinatus arises from all but the distal part of this fossa.

The infraspinous fossa, caudal to the spine, is triangular, with the apex at the neck. The infraspinatus arises from the infraspinous fossa.

The medial or costal surface has two areas. A small proximal and cranial rectangular area, the serrated face, serves as insertion for the serratus ventralis muscle. The large remaining part of the costal surface is the subscapular fossa, which is nearly flat and usually presents three straight muscular lines that converge distally. The subscapularis arises from the whole subscapular fossa.

The cranial border of the scapula is thin. Near the ventral angle the border is concave as it enters into the formation of the neck. The notch thus formed is the scapular notch. The dorsal end of the cranial border thickens and, without definite demarcation at the cranial angle, is continuous with the dorsal border.

The dorsal border extends from the cranial to the caudal angles. In life it is capped by a narrow band of cartilage, but in the dried specimen the cartilage is destroyed by ordinary preparation methods. The rhomboideus attaches to this border.

Just proximal to the ventral angle, the thick caudal border bears the infraglenoid tubercle, from which arise the teres minor and the long head of the triceps. The middle third of the caudal border of the scapula is broad and smooth; part of the subscapularis and the long head of the triceps arise from it. Somewhat less than a third of the dorsal segment of the caudal border is thick and gives rise to the teres major.

The ventral angle forms the expanded distal end of the scapula. The adjacent constricted part, the neck, is the segment of the scapula distal to the spine and proximal to the expanded part of the bone that forms the glenoid cavity. Clinically, the ventral angle is by far the most important part of the scapula, because it enters into the formation of the shoulder joint. The glenoid cavity articulates with the head of the humerus. Observe the shallowness of the cavity.

The supraglenoid tubercle is an eminence at the cranial part of the glenoid cavity. The tubercle shows a slight medial inclination on which a small tubercle, the coracoid process, can be distinguished. The coracobrachialis arises from the coracoid process, whereas the biceps brachii arises from the supraglenoid tubercle.

Humerus

The humerus (Fig. 2-5) is located in the arm, or brachium. This bone enters into the formation of both the shoulder joint and the elbow joint. The shoulder joint is formed by the articulation of the scapula and humerus; the elbow joint is formed by the articulation of the radius and ulna with each other and with the humerus. The proximal extremity of the humerus includes the head, neck, and the greater and lesser tubercles. The distal extremity, the condyle, includes the trochlea, capitulum, and the radial and olecranon fossae, which communicate proximal to the trochlea through the supratrochlear foramen. The medial and lateral epicondyles are situated on the sides of the condyle. The body of the humerus lies between the two extremities.

The head of the humerus is the part that articulates with the scapula. It presents more than twice the area of the glenoid cavity of the scapula and is elongated sagittally. Although the shoulder joint is a typical ball-and-socket joint, it normally undergoes only flexion and extension. The intertubercular groove begins at the cranial end of the articular area. It lodges the tendon of origin of the biceps brachii and is deflected toward the median plane by the greater tubercle, which forms the craniolateral part of the proximal extremity. The greater tubercle is convex at its summit and, in most breeds, higher than the head. It is continued distally in the body of the humerus by the crest of the greater tubercle. The greater tubercle receives the insertions of the supraspinatus and the infraspinatus and part of the deep pectoral. Between the head of the humerus and the greater tubercle are several foramina for the transmission of vessels. The infraspinatus is inserted on the smooth facet on the lateral side of the greater tubercle. The lesser tubercle lies on the medial side of the proximal extremity of the humerus, caudal to the intertubercular groove. It is not as high or as large as the greater tubercle. The subscapularis attaches to its proximal border. The neck of the humerus is not distinct except caudally. It is the line along which the head and parts of the tubercles have fused with the body.

The cranial surface of the humerus is distinct in the middle third of the body, where it furnishes attachment for the brachiocephalicus and part of the pectorals. Distally it fades but may be considered to continue to the medial lip of the trochlea. On the proximal third of the cranial border are two ridges. They continue to the cranial and caudal parts of the greater tubercle. The ridge that extends proximally in a craniomedial direction is the crest of the greater tubercle and is also the cranial border of the bone. This forms part of the area of insertion of the pectorals and the cleidobrachialis.

The ridge extending to the caudal part of the greater tubercle is on the lateral surface of the humerus. Distally it is thickened to form the deltoid tuberosity. The deltoideus inserts here. From this tuberosity to the caudal part of the greater tubercle, the ridge forms the prominent tricipital line. The lateral head of the triceps arises from this line. The teres minor inserts on the tuberosity of the teres minor adjacent to the proximal extremity of the tricipital line. The smooth brachialis groove is on the lateral surface of the body. The brachialis, which originates in the proximal part of the groove, spirals around the bone in the groove so that distally it lies on the craniolateral surface. Distal to this groove is the thick lateral supracondylar crest. The extensor carpi radialis and part of the anconeus attach here. The crest extends distally to the lateral epicondyle.

The caudal surface is smooth and rounded transversely and ends in the deep olecranon fossa.

The crest of the lesser tubercle crosses the proximal end of the medial surface and ends distally at the teres major tuberosity. The teres major and latissimus dorsi are inserted on this tuberosity. Caudal and proximal to this, the medial head of the triceps arises and the coracobrachialis is inserted. Approximately the middle third of the medial surface is free of muscular attachment and is smooth.

The distal end of the humerus, including its articular areas and the adjacent fossae, is the humeral condyle. The articular surface is divided unevenly by a low ridge. The large area medial to the ridge is the trochlea, which articulates with both the radius and the ulna and extends proximally into the adjacent fossae. The articulation with the trochlear notch of the ulna is one of the most stable hinge joints (ginglymus) in the body. The small articular area lateral to the ridge is the capitulum, which articulates only with the head of the radius.

The lateral epicondyle is smaller than the medial one and occupies the enlarged distolateral end of the humerus proximal to the capitulum. It gives origin to the common digital extensor, lateral digital extensor, ulnaris lateralis, and supinator. The lateral collateral ligament of the elbow also attaches here. The lateral supracondylar crest extends proximally from this epicondyle and is the origin for the extensor carpi radialis.

The medial epicondyle is the enlarged distomedial end of the humerus proximal to the trochlea. Its caudal projection deepens the olecranon fossa. The anconeus arises from this projection. The elevated portion of the medial epicondyle serves as origin for flexor carpi radialis, flexor carpi ulnaris, pronator teres, and the superficial and deep digital flexor muscles. The medial collateral ligament of the elbow also attaches here.

The olecranon fossa is a deep excavation of the caudal part of the humeral condyle. It receives the anconeal process of the ulna during extension of the elbow. On the cranial surface of the humeral condyle is the radial fossa, which communicates with the olecranon fossa by an opening, the supratrochlear foramen. No soft structures pass through this foramen.

Radius

The radius and ulna are the bones of the antebrachium, or forearm. It is important to know that they cross each other obliquely so that the proximal end of the ulna is medial and the distal end is lateral to the radius. The radius (Fig. 2-6), the shorter of the two bones of the forearm, articulates proximally with the humerus and distally with the carpus. It also articulates with the ulna, proximally by its caudal surface and distally near its lateral border.

The proximal extremity consists of head, neck, and tuberosity. The head of the radius, like the whole bone, is widest medial to lateral. It forms proximally an oval, depressed articular surface, the fovea capitis, which articulates with the capitulum of the humerus. The smooth caudal border of the head is the articular circumference for articulation with the radial notch of the ulna. The small radial tuberosity lies distal to the neck on the medial border of the bone. The biceps brachii and brachialis insert in part on this tubercle.

The body of the radius is compressed so that it possesses cranial and caudal surfaces and medial and lateral borders. It is slightly convex cranially. At the carpal end, the body blends without sharp demarcation with the enlarged distal extremity. The caudal surface of the radius is roughened and slightly concave. It has a ligamentous attachment to the ulna. Distally it broadens and becomes the expanded caudal surface of the distal extremity. The cranial surface of the radius, convex transversely, is relatively smooth throughout.

The distal extremity of the radius is the trochlea. Its carpal articular surface is concave. On the lateral surface of the distal extremity is the ulnar notch, a slightly concave area with a facet for articulation with the ulna. The medial surface of the distal extremity ends in a rounded projection, the styloid process. The medial collateral ligament of the carpus attaches proximal to the styloid process. The cranial surface of the distal extremity presents three distinct grooves. The most medial groove, which is small, short, and oblique, contains the tendon of the abductor digiti I longus. The middle and longest groove, extending proximally on the shaft of the radius, is for the extensor carpi radialis. The most lateral of the grooves on this surface is wide and of variable distinctness. It contains the tendon of the common digital extensor.

Ulna

The ulna (Fig. 2-7) is located in the caudal part of the forearm. It exceeds the radius in length and is irregular in shape and generally tapers from its proximal to its distal end. Proximally the ulna is medial to the radius and articulates with the trochlea of the humerus by the trochlear notch and with the articular circumference of the radius by the radial notch. This forms the elbow. Distally the ulna is lateral and articulates with the radius medially and with the ulnar and accessory carpal bones distally.

The proximal extremity is the olecranon, which includes the olecranon tuber and the anconeal process. It serves as a lever arm for the extensor muscles of the elbow. It is four sided, laterally compressed, and medially inclined. Its proximal end, the olecranon tuber, is grooved cranially and enlarged and rounded caudally. The triceps brachii, anconeus, and tensor fasciae antebrachii attach to the caudal part of the olecranon. The ulnar portions of the flexor carpi ulnaris and deep digital flexor arise from its medial surface.

The trochlear notch is a smooth, vertical, half-moon–shaped concavity facing cranially. The whole trochlear notch articulates with the trochlea of the humerus. At its proximal end a sharp-edged, slightly hooked anconeal process fits into the olecranon fossa of the humerus when the elbow joint is extended. At the distal end of the notch are the medial and lateral coronoid processes, which articulate with the humerus and radius. The medial coronoid process is larger. Between these processes is the radial notch for articulation with the articular circumference of the radius.

The body of the ulna is three sided in its middle third; proximal to this the bone is compressed laterally, whereas the distal third gradually loses its borders, becomes irregular, and is continued by the pointed distal extremity. The ulnar tuberosity is a small, elongated eminence on the medial surface of the bone at its proximal end, just distal to the medial coronoid process. The biceps brachii and the brachialis insert on this eminence. The interosseous border is distinct, rough, and irregular, especially at the junction of the proximal and middle thirds of the bone, where a large, expansive, but low eminence is found. This eminence indicates the place of articulation with the radius by means of a heavy ligament. Frequently, a vascular groove medial to the crest marks the position of the caudal interosseous artery. This groove is most conspicuous in the middle third of the ulna. The body shows a distinct caudal concavity.

The distal extremity of the ulna is the head with its prominent styloid process. A part of this process articulates with the ulnar and accessory carpal bones. The head articulates medially with the radius.

Carpal Bones

The term carpus (Figs. 2-8 to 2-10) is used to designate that part of the extremity between the antebrachium and metacarpus that includes all the soft structures as well as the bones. The carpus includes seven small, short irregular bones arranged into two rows. These are most conveniently studied on radiographs. The proximal row contains three bones. The largest of these, the intermedioradial carpal (often referred to as the radial carpal), is on the medial side and articulates proximally with the radius. The ulnar carpal is the lateral member of the proximal row. Its palmar portion projects distally palmar and lateral to the fourth carpal bone. The accessory carpal, the palmar member, is a short rod of bone that articulates with the styloid process of the ulna and the ulnar carpal bone and serves as a lever arm for some of the flexor muscles of the carpus. The distal row consists of four bones numbered from the medial to the lateral side. From the smallest on the medial side, these are the first, second, third, and fourth carpal bones. The fourth carpal bone is the largest and articulates with the base of the fourth and fifth metacarpals.

Metacarpal Bones

The metacarpus (Figs. 2-8 and 2-9) contains five bones. The metacarpal bones are long bones in miniature, possessing a slender body, or shaft, and large extremities. The proximal extremity is the base, and the distal one is the head. The metacarpals, like the carpals and digits, are numbered from medial to lateral. Proximally all articulate principally with the corresponding carpal bones, except the fifth, which articulates with the fourth carpal. Distally all articulate with the corresponding proximal phalanges. Note the sagittal ridge on the head for articulation with the sagittal groove in the base of the corresponding proximal phalanx. The interosseous muscles largely fill the intermetacarpal spaces palmar to the metacarpal bones.

The first metacarpal bone is atypical. It is a vestigial structure, but unlike the first metatarsal bone in the hindpaw, it is constantly present.

Phalanges

In the forepaw there are three phalanges for each of the four main digits (Figs. 2-8 and 2-9); the first digit, or pollex, a dewclaw, has two phalanges. Each proximal and middle phalanx has a proximal base, a body, and a distal head.

On the distal phalanx, a thin shelf of bone, the ungual crest, overlaps the claw and forms a band of bone around the proximal portion of the claw. The ungual process is a curved conical extension of the distal phalanx into the claw. The rounded dorsal part of the base is the extensor process on which the common digital extensor tendon is inserted. A small process on the palmar surface is the flexor tubercle for insertion of the deep digital flexor tendon.

Two proximal sesamoid bones are located in the interosseous tendons on the palmar surface of each metacarpophalangeal joint (digits II–V). Four small dorsal sesamoid bones (none for the first digit) are embedded in the common digital extensor tendons as they pass over the metacarpophalangeal joints (Figs. 2-8 and 2-9).

Superficial Structures of the Thorax

General study of the ventral surface of the trunk should be made before the dissection of the thoracic region is begun. Find the umbilicus. This is represented by a scar that may be either flat or slightly raised and is located on the midventral line, one third to one fourth of the distance from the xiphoid cartilage to the scrotum or vulva. In most dogs it is hidden by hair. The umbilicus is irregularly oval and may be from a few millimeters to a centimeter in length. The umbilicus serves as a landmark in abdominal surgery. Notice that the hair over a large area around the umbilicus slants toward it, thus forming a vortex.

Pick up a fold of skin, or common integument, which consists of an outer thin epithelium, the epidermis, and an underlying thicker layer of connective tissue, the dermis. Skin thickness varies on different parts of the body, depending on the extent of the dermis. Notice that the skin is thickest in the neck region, thinner over the sternum, and thinnest on the ventral surface of the abdomen; also notice that the skin of the dorsum of the neck and thorax is loosely attached.

The mammae vary in number from 8 to 12, but 10 is average. They are situated in two rows, usually opposite each other. The number is usually reduced in the smaller breeds.

When 10 glands are present, the cranial four are the thoracic mammae, the following four are the abdominal mammae, and the caudal two are the inguinal mammae. When the abdominal and inguinal mammae are maximally developed, the glandular tissue in each row appears to form a continuous mass. The mammae lie in areolar connective tissue and are not fused to the body wall. The cranial pair of thoracic mammae are smaller than the other pair. Each mamma has a papilla, or nipple, that is partly hairless and contains about 12 openings, but these vary and are difficult to see if the animal is not lactating.

The costal cartilages of the tenth, eleventh, and twelfth ribs unite with each other to form the costal arch. Palpate this arch and the caudal border and free end of the last or thirteenth rib. This rib does not attach to the costal arch and is therefore called a “floating rib.”

Make a midventral incision through the skin only from the cranial end of the neck to the umbilicus (Fig. 2-11). From the umbilicus extend a transverse incision to the mid-dorsal line on the left side. From a point on the midventral incision directly opposite the arm, extend a transverse incision to the left elbow. Make a complete circular incision through the skin around the elbow. Extend a third transverse incision from the cranial end of the midventral incision to the mid-dorsal line on the left side. This should pass just caudal to the ear. Carefully reflect the skin of the thorax and neck to the mid-dorsal line. The skin will be intimately fused with the thin underlying cutaneous muscle over the neck, thorax, and abdomen. The muscle should be left on the specimen as far as is possible.

The subcutaneous tissue that now confronts the dissector is composed of areolar tissue and fascia. Areolar tissue appears as a thin layer of loose, irregularly arranged connective tissue that often contains fat. Fascia is a denser, more regularly arranged thin layer of connective tissue. It is more fibrous, and it envelops the body beneath the skin and encloses individual muscles or groups of muscles. The superficial fascia is deep to the areolar tissue, forming the deep portion of the subcutaneous tissue that covers the entire body. It blends with the deep fascia, which is more firmly attached to the muscle that it encloses. These are not always easily distinguished from each other. The areolar tissue is often distended with embalming fluid. (Clinical subcutaneous injections are made into this tissue.) When the fatty areolar tissue and fascia with vessels and nerves are removed from a muscle, the muscle is said to be “cleaned.”

All muscles have attachments. In most instances the more proximal attachment, the part that moves the least, is considered the origin. The insertion is the more distal attachment, or the part that moves the most. The origin is usually a direct attachment of the muscle cells to the bone. The insertion often is by a tendon or aponeurosis extending from the muscle cells to the bone. A tendon consists of dense, regularly arranged fibrous connective tissue organized into a small, well-defined bundle. An aponeurosis has the same consistency as a tendon, but the fibrous tissue is arranged as a thin sheet of tissue. A ligament is dense fibrous connective tissue between bones, although the term is also used for a variety of thin fibrous connections between organs or between an organ and the body wall.

Read before you dissect! In many instances, during the study of muscles, a description of a specific muscle will be given before the instructions for dissection. At no time should muscles be removed or even transected without instructions. In each instance clean the exposed surface of the muscle being described, isolate its borders, and verify its origin and insertion. If the muscle is to be “transected,” free it from underlying structures first. As each muscle is defined, visualize its attachments and position on the skeleton and understand its function.

The cutaneus trunci (Figs. 2-15 and 2-17) is a thin sheet of muscle that covers most of the dorsal, lateral, and ventral walls of the thorax and abdomen. It has no direct bony attachments. It is more closely applied to the skin than to underlying structures and is often reflected with the skin before being observed. Like all cutaneous muscles, it is developed in the superficial fascia of the thorax and abdomen. Caudal to the shoulder the fibers sweep obliquely toward the axilla; farther caudally they are principally longitudinal and arise from the superficial fascia over the pelvic region.

The attachments of the cutaneus trunci are the superficial fascia of the trunk and the skin. The muscle sends a fasciculus to the medial side of the forelimb; caudal and ventral to this the fibers fray out over the deep pectoral muscle. The cutaneus trunci twitches the skin. It is innervated by the lateral thoracic nerve. In the male dog there is a distinct development of this muscle adjacent to the ventral midline caudal to the xiphoid. This is the preputial muscle, which passes caudally and radiates into the prepuce, forming an arch with the muscle of the opposite side. It functions to support the cranial end of the prepuce during the nonerect state and to pull the prepuce back over the glans penis after erection and protrusion.

Sever the axillary and ventral attachments of the cutaneus trunci and reflect it dorsally. Caution: Beneath the thin cutaneus trunci is the thicker latissimus dorsi muscle, which should be left in place on the lateral side of the trunk.

Extrinsic Muscles of the Thoracic Limb and Related Structures

The extrinsic muscles of the thoracic limb are those that attach the limb to the axial skeleton; the intrinsic muscles extend between the bones that compose the limb itself. Extrinsic muscles of the thoracic limb are as follows:

In the ventral thoracic region are the superficial and deep pectoral muscles, which extend between the sternum and the humerus. Thoroughly clean these muscles. In thin specimens this will require little dissecting. In pregnant or lactating bitches, it will require reflecting the two thoracic mammae caudally, and in fat specimens the forelimb will probably have to be manipulated so that the borders of the muscles are clearly discernible before cleaning. Always clean the extremities of a muscle as well as the middle part. Actually see and feel the attachments. Visualize the muscle’s position and action on the skeleton and attempt to palpate it on a live dog.

ORIGIN: The first two sternebrae and usually a part of the third; the fibrous raphe between adjacent muscles.

INSERTION: The whole crest of the greater tubercle of the humerus.

ACTION: To adduct the limb when it is not bearing weight or to prevent the limb from being abducted when bearing weight.

INNERVATION: Cranial pectoral nerves (C7, C8).

ORIGIN: The ventral part of the sternum and the fibrous raphe between fellow muscles; the deep abdominal fascia in the region of the xiphoid cartilage (the caudal end of the sternum).

INSERTION: The major portion partly muscular, partly tendinous on the lesser tubercle of the humerus; an aponeurosis to the greater tubercle and its crest; the caudal part to the medial brachial fascia.

ACTION: When the limb is advanced and fixed: to pull the trunk cranially and to extend the shoulder joint. When the limb is not supporting weight: to draw the limb caudally and flex the shoulder joint. To adduct the limb.

INNERVATION: Caudal pectoral nerves (C8, T1).

The superficial fascia of the neck is continued on the head as the superficial fascia of the various regions of the head. Caudally it becomes continuous with the superficial brachial and pectoral fasciae. Some of the fascia is also continued into the axillary space. Notice that the external jugular vein is completely wrapped by it. Save this vein for future orientation. The cutaneous muscles of the neck are completely enveloped by the fascia. Only the platysma will be dissected.

The platysma (Fig. 5-20, A) is the best developed of the cutaneous muscles of the neck and head. Its fibers sweep cranioventrally over the dorsal part of the neck and over the lateral surface of the face. The muscle may have been reflected with the skin. Its insertion will be seen when the head is dissected.

Transect the cervical part of the cleidocephalicus to expose the extent of the mastoid part below. Note that the mastoid part runs toward the head deep to the sternocephalicus. Transect the mastoid part of the muscle and search for the clavicle by inserting a finger on the medial side of the clavicular intersection.

ATTACHMENTS: All attachments are movable, but the clavicle or clavicular intersection is considered the origin for purposes of naming the muscles. The cleidobrachialis attaches to the distal end of the cranial border of the humerus. There is also a significant fascial tie into the axilla. The cervical part of the cleidocephalicus attaches to the cranial half of the mid-dorsal fibrous raphe and sometimes to the nuchal crest of the occipital bone. The mastoid part of the cleidocephalicus attaches to the mastoid process of the temporal bone with the sternomastoideus muscle.

ACTION: To advance the limb; to extend the shoulder joint and draw the neck and head to the side.

INNERVATION: Accessory nerve and ventral branches of cervical spinal nerves.

ORIGIN: The first sternebra or manubrium.

INSERTION: The mastoid part of the temporal bone and the nuchal crest of the occipital bone.

ACTION: To draw the head and neck to the side.

INNERVATION: Accessory nerve and ventral branches of cervical spinal nerves.Transect the left sternocephalicus close to the manubrium and reflect it. The sternohyoid and sternothyroid muscles are both covered by the deep fascia of the neck and lie dorsal to the sternocephalicus at their origin.

The sternohyoideus (Figs. 2-12 and 5-32) lies on the trachea, covered by the sternocephalicus caudally. A midventral groove indicates the separation of right and left muscles.

ORIGIN: The first sternebra and the first costal cartilage.

INSERTION: The basihyoid bone.

ACTION: To pull the tongue and larynx caudally.

INNERVATION: Ventral branches of cervical spinal nerves.

The sternothyroideus (Figs. 2-12 and 5-32) is covered at its origin by the sternohyoideus. The sternothyroideus inserts on the lateral surface of the thyroid cartilage. The left muscle is bounded dorsally by the esophagus and medially by the trachea. Notice that a tendinous intersection runs across the muscle 3 or 4 cm cranial to its origin. It is at this level that the sternohyoideus separates from the sternothyroideus.

ORIGIN: The first costal cartilage.

INSERTION: The caudolateral surface of the thyroid cartilage.

ACTION: Same as the sternohyoideus—to draw the larynx and tongue caudally.

INNERVATION: Ventral branches of cervical spinal nerves.

ATTACHMENTS: The distal end of the spine of the scapula; cranially, the transverse wing of the atlas.

ACTION: To advance the limb or flex the neck laterally.

INNERVATION: Accessory nerve.

The deep fascia of the neck is a strong wrapping that extends under the sternocephalicus, omotransversarius, and cleidocephalicus muscles. It covers the sternohyoideus and sternothyroideus ventrally and surrounds the trachea, thyroid gland, larynx, and esophagus. The deep fascia that covers the common carotid artery, vagosympathetic nerve trunk, internal jugular vein, and tracheal lymphatic trunk is the carotid sheath. Locate these structures in the carotid sheath between the omotransversarius dorsally and the sternothyroideus ventrally. The deep fascia of the neck continues dorsally and laterally to invest the deep cervical muscles.

The supraspinous ligament (Fig. 2-84) connects the dorsal aspects of all vertebral spines except the cervical vertebrae (do not dissect at this time). The nuchal ligament, composed predominantly of yellow elastic fibrous tissue, extends from the spine of the first thoracic vertebra to the spine of the axis. The median raphe of the neck is a longitudinal fibrous septum between right and left epaxial muscles dorsal to the nuchal ligament. It serves as the attachment for many of the cervical muscles. Observe these during the dissection of the following muscles.

ORIGIN: The median raphe of the neck and the supraspinous ligament from the level of the third cervical vertebra to the level of the ninth thoracic vertebra.

INSERTION: The spine of the scapula.

ACTION: To elevate and abduct the forelimb.

INNERVATION: Accessory nerve.

ORIGIN: The nuchal crest of the occipital bone; the median fibrous raphe of the neck; the spinous processes of the first seven thoracic vertebrae.

INSERTION: The dorsal border and adjacent surfaces of the scapula.

ACTION: To elevate the forelimb and draw the scapula against the trunk.

INNERVATION : Ventral branches of cervical and thoracic spinal nerves.

ORIGIN: The thoracolumbar fascia from the spinous processes of the lumbar and the last seven or eight thoracic vertebrae; a muscular attachment to the last two or three ribs.

INSERTION: The teres major tuberosity of the humerus and the teres major tendon.

ACTION: To draw the free limb caudally as in digging; to flex the shoulder joint.

INNERVATION: Thoracodorsal nerve (C7, C8, T1).

The thoracolumbar fascia is deep fascia of the trunk. It arises from the supraspinous ligament and spines of the thoracic and lumbar vertebrae; covers the muscles of the vertebrae, ribs, and abdomen; and fuses with the opposite fascia on the ventral midline along a median fibrous raphe, the linea alba. The thoracolumbar fascia serves as an attachment for numerous muscles. It will be dissected with the abdominal muscles.

ORIGIN: The transverse processes of the last five cervical vertebrae and the first seven or eight ribs ventral to their middle.

INSERTION: The dorsomedial third of the scapula (serrated face).

ACTION: To support the trunk and depress the scapula.

INNERVATION: Ventral branches of cervical spinal nerves and the long thoracic nerve (C7).

Lateral Muscles of the Scapula and Shoulder

ORIGIN: The spine and acromial process of the scapula.

INSERTION: The deltoid tuberosity.

ACTION: To flex the shoulder.

INNERVATION: Axillary nerve.

ORIGIN: The infraspinous fossa.

INSERTION: A small, circumscribed area on the lateral side of the greater tubercle of the humerus.

To extend and laterally stabilize or flex the shoulder joint, depending on the degree of extension or position of the joint when the muscle contracts. To abduct the shoulder joint and to rotate the shoulder laterally. To prevent medial rotation when weight bearing and provide lateral stability to the shoulder joint.

INNERVATION: Suprascapular nerve.

ORIGIN: The infraglenoid tubercle and distal third of the caudal border of the scapula.

INSERTION: The teres minor tuberosity of the humerus.

ACTION: To flex the shoulder joint, rotate the shoulder laterally, and prevent medial rotation when bearing weight.

INNERVATION: Axillary nerve.

ORIGIN: The supraspinous fossa.

INSERTION: The greater tubercle of the humerus, by a thick tendon.

ACTION: To extend and laterally stabilize the shoulder joint.

INNERVATION: Suprascapular nerve.

Medial Muscles of the Scapula and Shoulder

ORIGIN: The subscapular fossa.

INSERTION: The lesser tubercle of the humerus.

ACTION: To adduct, extend, and medially stabilize the shoulder joint. To rotate the shoulder medially and prevent lateral rotation when bearing weight.

INNERVATION: Subscapular nerve. Considerable support is provided to the shoulder joint by the subscapularis muscle medially, the teres minor and infraspinatus muscles laterally, and the supraspinatus cranially. Luxation will not occur without injury to the joint capsule and its glenohumeral ligaments (Fig. 2-29).

ORIGIN: The caudal angle and adjacent caudal border of the scapula; the caudal surface of the subscapularis.

INSERTION: The teres major tuberosity of the humerus.

ACTION: Flex the shoulder, rotate the shoulder medially, and prevent lateral rotation when weight bearing.

INNERVATION: Axillary nerve.

ORIGIN: The coracoid process of the scapula.

INSERTION: The crest of the lesser tubercle of the humerus proximal to the teres major tuberosity.

ACTION: To adduct, extend, and stabilize the shoulder joint.

INNERVATION: Musculocutaneous nerve.

Os Coxae

The cranial border is arciform and usually roughened and is more commonly known as the iliac crest. It is thin but gradually increases in thickness dorsally. The angle of junction of the iliac crest with the ventral border is known as the cranial ventral iliac spine, which provides a place of origin for both bellies of the sartorius and a part of the tensor fasciae latae. The tuber coxae is composed of the cranial ventral iliac spine and the adjacent part of the ventral border of the wing of the ilium. The rest of the ventral border is concave. It ends in the lateral area for the rectus femoris (Fig. 2-33) just cranial to the acetabulum.

The dorsal border of the ilium is broad and massive. The junction of the dorsal border with the iliac crest forms an obtuse angle that is a rounded prominence, the cranial dorsal iliac spine. Caudal to the cranial dorsal iliac spine is the wide but blunt caudal dorsal iliac spine. The two spines and intervening bone make up the tuber sacrale, which occupies nearly half the length of the dorsal border of the ilium. The caudal half of the dorsal border is gently concave. It forms the greater ischiatic notch and also helps form the ischiatic spine, which is dorsal to the acetabulum.

The external or gluteal surface (Fig. 2-33) of the wing of the ilium is nearly flat caudally and concave cranially, where it is limited by the iliac crest. The dorsal part of this concave area is bounded by a heavy ridge, the tuber sacrale. The gluteal surface is rough ventrocranially. The middle gluteal and a portion of the deep gluteal attach here.

The internal or sacropelvic surface (Fig. 2-34) of the wing of the ilium presents a smooth, nearly flat area that provides attachment for the iliocostalis, longissimus, and the quadratus lumborum muscles. The auricular surface is rough and articulates with a similar surface of the sacrum, forming the sacroiliac joint. The arcuate line is located along the ventromedial edge of the sacropelvic surface of the body of the ilium and runs from the auricular surface to the iliopubic eminence of the pubis. The tendon of the psoas minor attaches along the medial aspect of this line.

The body of the ischium is the part lateral to the obturator foramen. The ischiatic spine is a rounded crest dorsal to the acetabulum, where the body of the ischium meets the ilium. The coccygeus attaches here. Caudal to this spine the border of the ischium is depressed and marked by a series of low ridges produced by the tendon of the internal obturator. This area is known as the lesser ischiatic notch (Figs. 2-33, 2-34). The gemelli arise from the lateral surface adjacent to the lesser ischiatic notch.

The ramus of the ischium is the thin and wide medial part of the ischium. It is bounded laterally by the obturator foramen and blends caudally with the body of the ischium. The ramus meets its fellow at the symphysis and is fused with the pubis cranially. The ischiatic table is the flat portion where the ramus meets the body (Fig. 2-34). It faces dorsally and is the site of origin of the internal obturator muscle. The quadratus femoris and the external obturator arise from its ventral surface. The ischiatic arch is formed by the medial portion of the caudal border of each ischium.

The acetabulum (Figs. 2-32, 2-33) is a cavity that receives the head of the femur. Its articular surface is semilunar and is composed of parts of the ilium, ischium, and, in young animals, the acetabular bone (Fig. 2-32). In the adult the acetabular bone is fused imperceptibly with the pubis, ischium, and ilium. The circumference of the articular surface is broken at the caudomedial part by the acetabular notch. The acetabular fossa is formed by the ischium and the acetabular bone. The ligament of the head of the femur attaches in this fossa. The fossa and the notch are the nonarticular parts of the acetabulum. The two sides of the notch are connected by the transverse acetabular ligament.

The obturator foramen is closed in life by the obturator membrane and the external and internal obturator muscles that the membrane separates.

Femur

The femur (Figs. 2-35, 2-36), or thigh bone, is the largest bone in the body. The flexor angle of the hip is about 110 degrees. The flexor angle at the stifle is from 130 to 135 degrees.

The femur is a typical long bone with a cylindrical body and two expanded extremities. The proximal extremity presents on its medial side a smooth, nearly hemispherical head, most of which is articular except for a small shallow fossa beginning near the middle of the head and usually extending to its caudomedial margin. This fossa is the fovea capitis femoris, to which the ligament of the head of the femur attaches. The head is attached to the medial part of the proximal extremity by the neck of the femur. The neck is distinct but short and provides attachment for the joint capsule. The greater trochanter, the largest eminence of the proximal extremity, is located directly lateral to the head. To it attach the middle gluteal and deep gluteal. The trochanteric fossa is a deep cavity medial to the greater trochanter. The gemelli and the external and internal obturators insert in this fossa. The lesser trochanter, a pyramidal projection at the proximal end of the medial side of the body of the femur, serves for the insertion of the iliopsoas. A ridge of bone extends from the summit of the greater trochanter to the lesser trochanter. This, the intertrochanteric crest, represents the caudolateral boundary of the trochanteric fossa. The quadratus femoris inserts on the crest at the level of the lesser trochanter. The third trochanter is poorly developed. It appears at the base of the greater trochanter as a small, rough area on which the superficial gluteal inserts. The third and lesser trochanters are located in about the same transverse plane. The vastus parts of the quadriceps femoris attach to the smooth proximal cranial part of the femur.

The body of the femur is slightly convex cranially. Viewed cranially, the body presents a smooth, rounded surface. The caudal surface is rough and is limited by medial and lateral lips. The lips, closest together in the middle of the body, diverge as they approach each extremity. The proximal part of the medial lip ends in the lesser trochanter, the distal part at the medial supracondylar tuberosity. The proximal part of the lateral lip ends in the third trochanter, the distal part at the lateral supracondylar tuberosity. The adductor inserts on most of the caudal rough surface, whereas a tendon extends from the pectineus to the distal part of the medial lip, where the semimembranosus also attaches.

The distal extremity of the femur presents several articular surfaces. The trochlea, with ridges, is the smooth groove on the craniodistal part of the bone for articulation with the patella. The medial trochlear ridge is usually thicker than the lateral. The patella is a sesamoid in the tendon of insertion of the large quadriceps femoris that extends the stifle. It aids in the protection of the tendon and the joint, but its chief purpose is redirection of the tendon of insertion of the quadriceps. The trochlea of the femur is continuous with the condyles, which articulate, both directly and through fibrocartilaginous menisci, with the tibia. The medial and lateral condyles are separated from each other by the intercondylar fossa, a deep, wide space. The two condyles are similar in shape and surface area. Each is convex transversely and longitudinally. At the depth of the intercondylar fossa the cruciate ligaments attach. On the caudodorsal aspect of each femoral condyle is a facet on which a sesamoid bone (fabella) rests. The medial and lateral fabellae are in the tendons of origin of the medial and lateral heads of the gastrocnemius muscle. Proximal to these sesamoid facets are the medial and lateral supracondylar tuberosities from which the gastrocnemii arise. The superficial digital flexor also arises from the lateral tuberosity. The popliteal surface is a large, flat, triangular area on the caudal surface of the distal extremity proximal to the condyles and intercondylar fossa. The medial and lateral epicondyles are rough areas on each side, proximal to the condyles. They serve for the attachment of the collateral ligaments of the stifle. The lateral epicondyle also gives rise to the popliteus. The small extensor fossa is located on the lateral epicondyle at the junction of the lateral condyle and the lateral lip of the trochlea; from it arises the long digital extensor. The semimembranosus is inserted just proximal to the medial epicondyle.

Tibia

The tibia (Fig. 2-37), the shin or leg bone, has a proximal articular surface that flares out transversely and is also broad craniocaudally. It is wider than the distal end of the femur, with which it articulates, and is formed largely by two relatively flat condyles. The medial condyle is separated from the lateral condyle by the intercondylar eminence. Both condyles include the articular areas on their proximal surfaces and the adjacent nonarticular parts of the proximal extremity. The lateral condyle is particularly prominent. It possesses a facet on its lateral side for articulation with the head of the fibula and provides origin for part of the peroneus longus and cranial tibial muscles. A sesamoid bone in the tendon of origin of the popliteus (seen in radiographs) articulates with the caudolateral condyle of the tibia. The semimembranosus is inserted on the medial condyle. Two biconcave fibrocartilages, the menisci, fill part of the space between the apposed condyles of the femur and tibia, making the joint congruent. The intercondylar eminence consists of two small, elongated tubercles, which form its highest part, and a shallow intercondylar area. The cranial intercondylar area is a depression cranial to the eminence and in large part between the condyles. It affords attachment to the cranial parts of the menisci and the cranial cruciate ligament. The caudal intercondylar area occupies a place similar to that of the cranial area but caudal to the eminence. It provides attachment for the caudal part of the medial meniscus. The popliteal notch is caudal to the caudal intercondylar area and is located between the two condyles. The popliteal vessels pass through the notch. The tibial tuberosity is the large quadrangular process on the proximocranial surface of the tibia. The quadriceps femoris, the biceps femoris, and the sartorius attach to this tuberosity by means of the patella and patellar ligament. The tibial tuberosity is continued distally by the cranial border of the tibia. It inclines laterally on the body. The following muscles attach wholly or in part to the cranial border of the tibia: biceps femoris, semitendinosus, gracilis, and sartorius. The extensor groove is a small, smooth groove located at the junction of the lateral condyle and the tibial tuberosity. The long digital extensor passes through it.

The body is triangular proximally, nearly cylindrical in the middle, and four sided distally. The semitendinosus and gracilis are inserted on the proximal medial surface. The proximal third of the caudal surface serves for the insertion of the popliteus medially and for the origins of the deep digital flexor laterally.

The distal extremity of the tibia is quadrilateral in transverse section. The tibial cochlea, the articular surface, consists of two grooves that receive the ridges of the proximal trochlea of the talus. The medial part of the distal extremity of the tibia is the medial malleolus. The lateral surface of the distal extremity articulates with the fibula by a small facet. No muscles arise from the distal half of the tibia.

Fibula

The fibula (Figs. 2-35, 2-37) has proximal and distal extremities and an intermediate body. The proximal extremity, or head, articulates with the lateral condyle of the tibia. The distal extremity, the lateral malleolus, has two grooves that contain the tendons of the fibularis longus, fibularis brevis, and the lateral digital extensor. These grooves redirect the force of contraction. On the medial surface of the tibia is a distinct facet for articulation with the distolateral surface of the tibia and with the talus.

Tarsal Bones

The tarsus (Figs. 2-35, 2-38), between the metatarsals and the leg, is composed of seven tarsal bones and the related soft tissues. It is also called the hock. The bones are arranged in three irregular rows. The proximal row is composed of a long, laterally located calcaneus and a shorter, medially located talus. The talus has a trochlea on its proximal end with two ridges separated by a groove for articulation with the tibial cochlea. This is the tarsocrural joint where flexion and extension occur between the leg and hindpaw. The talus articulates with the calcaneus laterally and the central tarsal bone distally. The calcaneus articulates with the talus and the fourth tarsal bone.

The tuber calcanei is a traction process of the calcaneus that projects proximally and caudally. The extensor muscles of the hock insert on this process via the common calcanean tendon. On the medial side of the calcaneus is a bony process, the sustentaculum tali. The tendon of the lateral digital flexor glides over the plantar surface of this process.

The distal row consists of four bones. Three small bones, the first, second, and third tarsal bones, are located side by side and are separated from the proximal row by the central tarsal bone. The large fourth tarsal bone, which completes the distal row laterally, articulates with the calcaneus proximally. The fourth tarsal is as long as the combined lengths of the third and central tarsal bones against which it lies. The fourth tarsal bone is grooved on the distal half of its lateral surface for the passage of the tendon of the fibularis longus.

Metatarsal Bones

The metatarsal bones (Fig. 2-38) resemble the metacarpal bones except for the first, which may be divided, rudimentary, or absent.

Phalanges

The phalanges (Fig. 2-39) and sesamoids form the skeleton of the digit. Those of the hindpaw, or pes, are similar to those of the forepaw, or manus.

The first digit, or hallux, is frequently absent. When present, it is called a dewclaw and may vary from a fully developed digit, articulating with a normal first metatarsal bone, to a vestigial structure composed only of a terminal phalanx.

Caudal thigh

Medial thigh

Lateral pelvis (rump)

Caudal hip

Cranial thigh

Caudal leg

Caudal Muscles of the Thigh

This group consists of three primary muscles: biceps femoris, laterally; semitendinosus, caudally; and semimembranosus, medially. (A slender caudal crural abductor muscle is closely associated with the mediocaudal surface of the biceps [Figs. 2-41, 2-43].)

ORIGIN: The sacrotuberous ligament and the ischiatic tuberosity.

INSERTION: By means of the fascia lata and crural fascia to the patella, patellar ligament, and cranial border of the tibia; by means of the crural fascia to the subcutaneous part of the tibial body; the tuber calcanei.

ACTION: To extend the hip, stifle, and hock. The caudal part of the muscle flexes the stifle.

INNERVATION: Sciatic nerve.

ORIGIN: The ischiatic tuberosity.

INSERTION: The disto-cranial border of the tibia. The medial surface of the body of the tibia and the tuber calcanei by means of the crural fascia.

ACTION: To extend the hip, flex the stifle, and extend the hock.

INNERVATION: Sciatic nerve.

ORIGIN: The ischiatic tuberosity.

INSERTION: The distal medial lip of the caudal rough surface of the femur and the medial condyle of the tibia.

ACTION: To extend the hip. The part that attaches to the femur extends the stifle; the part that attaches to the tibia flexes or extends the stifle, depending on the position of the limb.

INNERVATION: Sciatic nerve.

Medial Muscles of the Thigh

ORIGIN: Cranial part—the crest of the ilium and the thoracolumbar fascia; caudal part—the cranial ventral iliac spine and the adjacent ventral border of the ilium.

INSERTION: Cranial part—the patella, in common with the rectus femoris of the quadriceps; caudal part—the cranial border of the tibia, in common with the gracilis.

ACTION: To flex the hip. The cranial part extends the stifle; the caudal part flexes the stifle.

INNERVATION: Femoral nerve.

ORIGIN: The pelvic symphysis by means of the symphysial tendon.

INSERTION: The cranial border of the tibia and, with the semitendinosus, the tuber calcanei.

ACTION: To adduct the limb, flex the stifle, and extend the hip and hock.

INNERVATION: Obturator nerve.

The femoral triangle (Fig. 2-79) is the shallow triangular space through which the femoral vessels run to and from the pelvic limb. It is located on the proximal medial surface of the thigh with its base at the abdominal wall. The triangle lies between the caudal belly of the sartorius cranially and the pectineus and adductor caudally. The iliopsoas and rectus femoris form the proximal lateral part of the triangle. The vastus medialis forms the distal lateral part. This triangle contains, among other structures, the femoral artery and vein. Remove the medial femoral fascia and adipose tissue that covers and fills in around the femoral artery and vein. Notice that the vein lies caudal to the artery. The pulse is usually taken from the femoral artery here.

ORIGIN: From the iliopubic eminence and the pubic tubercle via the prepubic tendon.

INSERTION: The distal end of the medial lip of the caudal rough face of the femur.

ACTION: To adduct the limb.

INNERVATION: Obturator nerve.

ORIGIN: The entire pelvic symphysis by means of the symphysial tendon, the adjacent part of the ischiatic arch, and ventral surface of the pubis and ischium.

INSERTION: The entire lateral lip of the caudal rough face of the femur.

ACTION: To adduct the limb and extend the hip.

INNERVATION: Obturator nerve.

Lateral Muscles of the Pelvis

ORIGIN: The tuber coxae and adjacent part of the ilium; the aponeurosis of the middle gluteal muscle.

INSERTION: The lateral femoral fascia.

ACTION: To tense the lateral femoral fascia, flex the hip, and extend the stifle.

INNERVATION: Cranial gluteal nerve.

ORIGIN: The lateral border of the sacrum and the first caudal vertebra, partly by means of the sacrotuberous ligament; the cranial dorsal iliac spine by means of the deep gluteal fascia.

INSERTION: The third trochanter.

ACTION: To extend the hip and abduct the limb.

INNERVATION: Caudal gluteal nerve.

ORIGIN: The crest and gluteal surface of the ilium.

INSERTION: The greater trochanter.

ACTION: To extend and abduct the hip and to rotate the pelvic limb medially.

INNERVATION: Cranial gluteal nerve.

ORIGIN: The body of the ilium; the ischiatic spine.

INSERTION: The cranial aspect of the greater trochanter.

ACTION: To extend and abduct the hip and to rotate the pelvic limb medially.

INNERVATION: Cranial gluteal nerve.

The articularis coxae (Fig. 2-50, B) is a small, spindle-shaped muscle lying on the craniolateral aspect of the hip joint capsule. It is covered by the deep gluteal muscle. It arises from the lateral surface of the ilium along with the rectus femoris and inserts on the neck of the femur. (No dissection is necessary.)

Caudal Hip Muscles

The four muscles of this group are important because of their proximity to the hip. They lie caudal to the hip and extend from the inner and outer surfaces of the ischium to the femur. All rotate the limb laterally. This action opposes the medial rotation by the gluteals so that the thigh moves in a sagittal plane at the hip.

ORIGIN: The symphysis pelvis and the dorsal surface of the ischium and pubis.

INSERTION: The trochanteric fossa of the femur.

ACTION: To rotate the pelvic limb laterally at the hip.

INNERVATION: Sciatic nerve.

ORIGIN: The lateral surface of the ischium, caudal to the acetabulum and ventral to the lesser ischiatic notch.

INSERTION: The trochanteric fossa.

ACTION: To rotate the pelvic limb laterally at the hip.

INNERVATION: Sciatic nerve.

ORIGIN: The ventral surface of the caudal part of the ischium.

INSERTION: Intertrochanteric crest.

ACTION: To extend the hip and rotate the pelvic limb laterally.

INNERVATION: Sciatic nerve.

ORIGIN: The ventral surface of the pubis and ischium.

INSERTION: The trochanteric fossa.

ACTION: To rotate the pelvic limb laterally.

INNERVATION: Obturator nerve.

Cranial Muscles of the Thigh

The patella, a sesamoid bone, is intercalated in the large tendon of insertion of the quadriceps. It articulates with the trochlea of the femur.

The patellar ligament extends from the patella to the tibial tuberosity. It is the distal end of the tendon of insertion of the quadriceps.

The rectus femoris (Figs. 2-42 through 2-44, 2-46, 2-47, 2-49) is the most cranial component of the quadriceps femoris and the only one to arise from the ilium. Proximally, it is circular in transverse section and passes between the vastus medialis and the vastus lateralis. Uncover the rectus femoris near its origin. Transect and reflect the proximal part. The rectus arises from the ilium cranial to the acetabulum and inserts on the tibial tuberosity. It is a flexor of the hip as well as an extensor of the stifle.

The vastus lateralis (Figs. 2-43, 2-44, 2-47, 2-49, 2-50) lies lateral and caudal to the rectus femoris, to which it is fused distally. The vastus lateralis is partly separated from the vastus intermedius by a scantily developed intermuscular septum. Notice that the vastus lateralis arises from the proximal part of the lateral lip of the caudal rough surface of the femur. It is inserted with the rectus femoris on the tibial tuberosity.

The vastus intermedius (Figs. 2-47, 2-49, 2-50) lies directly on the smooth cranial surface of the femur and is quite intimately fused with the other two vasti. It arises with the vastus lateralis, which covers it, from the lateral side of the proximal end of the femur. It inserts on the tibial tuberosity with the other members of the group.

The vastus medialis (Figs. 2-42, 2-45 through 2-47, 2-50) arises from the medial side of the proximal end of the cranial surface of the femur and the proximal end of the medial lip of the caudal rough surface. It is inserted with the other heads of the quadriceps on the tibial tuberosity.

ORIGIN: Rectus femoris—ilium; vasti muscles—proximal femur.

INSERTION: Tibial tuberosity.

ACTION: To extend the stifle and flex the hip (rectus).

INNERVATION: Femoral nerve.

ORIGIN: Psoas major—lumbar vertebrae; iliacus—cranioventral ilium.

INSERTION: Lesser trochanter.

ACTION: To flex the hip.

INNERVATION: Ventral branches of lumbar spinal nerves; femoral nerve.

Muscles of the Leg (Crus)

The muscles of the leg, or crus, the region between the stifle and hock, are divided into craniolateral and caudal groups. Note that the leg is not the pelvic limb. Remove the skin that remains on the distal part of the pelvic limb to the level of the proximal interphalangeal joints.

The superficial crural, tarsal, metatarsal, and digital fasciae are similar to the superficial fasciae of the corresponding regions of the forelimb. Cutaneous vessels and nerves course in the superficial fascia. One such vessel is the cranial branch of the lateral saphenous vein, which is used for venipuncture.

The medial and lateral femoral fasciae blend over the stifle and are continued distally in the leg as the deep crural fascia. The deep crural fascia covers the muscles of the leg and the free-lying surfaces of the crural skeleton. Septa from this fascia extend between the muscles to attach to the bone. Laterally, the fibers of the caudal branch of the biceps femoris radiate into it. Medially, the semitendinosus and gracilis are continuous with this fascia. These connections are located caudally where the crural fascia contributes to the common calcanean tendon.

Just proximal to the flexor surface of the tarsus, the deep crural fascia is thickened to form an oblique band of about 0.5 cm, the crural extensor retinaculum. As it stretches obliquely from the distal third of the fibula to the medial malleolus of the tibia, it binds down the tendons of the long digital extensor and the cranial tibial muscles.

The deep crural fascia decreases in thickness as it passes over the tarsus and becomes the deep tarsal fascia. A fibrous loop that attaches to the calcaneus wraps around the tendon of the long digital extensor. This is the tarsal extensor retinaculum. The deep fascia extends into the metatarsal and digital pads and closely joins these pads with the overlying skeletal and ligamentous parts. Make an incision through the cranial crural fascia and reflect it to the common calcanean tendon and the tibia.

Cervical Vertebrae

There are seven cervical vertebrae in most mammals. The atlas (Figs. 2-65, 2-67), or first cervical vertebra, is atypical in both structure and function. It articulates with the skull cranially. Its chief peculiarities are modified articular processes, a lack of a spinous process, and a reduction of its body half of when become the dens of the axis. The lateral shelflike transverse processes are thick, forming the wings of the atlas. These are united by the body (ventral arch) ventrally and arch dorsally. The two cranial articular foveae articulate with the occipital condyles of the skull to form the atlanto-occipital joint, of which the main movement is flexion and extension. The caudal articular foveae consist of two shallow glenoid cavities that form a freely movable articulation with the second cervical vertebra. Rotatory movement occurs at this atlantoaxial joint. Examine the caudal part of the dorsal surface of the body for the fovea dentis. This is concave from side to side and articulates with the dens of the second cervical vertebra. This articular area blends laterally with the caudal articular fovea. Besides the large vertebral foramen through which the spinal cord passes, there are two pairs of foramina in the atlas. The transverse foramina are actually short canals that pass obliquely through the transverse processes of the atlas and contain the vertebral artery and vein. The lateral vertebral foramina perforate the cranial part of the vertebral arch. The first cervical spinal nerves pass through these foramina.

The axis, or second cervical vertebra (Figs. 2-66, 2-67), presents an elongated, ridgelike spinous process as its most prominent characteristic. It is also unlike other vertebrae in that cranially the body projects forward in a peglike eminence, the dens. The ventral surface of the dens is articular, whereas the tip and dorsal surface may be rough because of ligamentous attachment. The cranial articular surface is located on the body and is continuous with the articular area of the dens. The caudal part of the vertebral arch has two articular processes that face ventrolaterally. At the root of the transverse process is the small transverse foramen. The cranial vertebral notch concurs with that of the atlas to form the first intervertebral foramen for the transmission of the second cervical nerve (Fig. 2-67). The caudal notch concurs with that of the third cervical vertebra to form the second intervertebral foramen, through which the third cervical nerve passes. Other features of this bone are similar to those of a typical vertebra.

The middle three cervical vertebrae (Fig. 2-68) differ little from a typical vertebra. The spinous processes are low but gradually increase in height from third to fifth. The transverse processes are two pronged and are perforated at the base by a transverse foramen.

The sixth cervical vertebra has a high spine and an expanded ventral lamina of the transverse process. The seventh cervical vertebra (Fig. 2-69) lacks transverse foramina and has the highest cervical spine.

The cranial articular processes of cervical vertebrae 3 to 7 face dorsally and cranially in apposition to the caudal articular processes of adjacent vertebrae. There is a prominent ventral crest on the caudal midventral aspect of the vertebral body.

Thoracic Vertebrae

There are 13 thoracic vertebrae (Fig. 2-70); the first nine are similar. The bodies of the thoracic vertebrae are short. The first through the tenth have a cranial and a caudal costal fovea on each side for rib articulation. Because the foveae of two adjacent vertebrae form the articular surface for the head of one rib, they are sometimes called demifacets. The body of the tenth vertebra frequently lacks caudal foveae, whereas the eleventh through the thirteenth have only one complete cranial costal fovea on each side. The head of the first rib articulates between the last cervical and first thoracic vertebrae. The tubercles of the ribs articulate with the transverse processes of the thoracic vertebrae of the same number in all instances.

The spine is the most conspicuous feature of each of the first nine thoracic vertebrae. The massiveness of the spines gradually decreases with successive vertebrae, but there is little change in length and direction until the seventh or eighth vertebra is reached. The spines then become progressively shorter and incline caudally through the ninth and tenth. The spine of the eleventh thoracic vertebra is nearly perpendicular to the long axis of that bone. This vertebra is designated the anticlinal vertebra. All spines caudal to the eleventh vertebra point cranially; all spines cranial to the eleventh vertebra point caudally (Fig. 2-75).

The transverse processes are short, blunt, and irregular. All contain costal foveae for articulation with the tubercles of the ribs.

The articular processes are located at the junctions of the pedicles and the laminae. The cranial pair are nearly confluent at the median plane on thoracic vertebrae 3 through 10. Similar to cervical vertebrae 3 through 7, the cranial articular surfaces of thoracic vertebrae 1 through 10 face dorsally and slightly craniomedially. Because the caudal articular processes articulate with the cranial ones of the vertebra behind, they are similar in shape but face in the opposite direction. The joints between thoracic vertebrae 10 to 13 are conspicuously modified because the articular surfaces of the caudal articular processes are located on the lateral side and the articular surfaces of the cranial articular processes of thoracic vertebrae 11 through 13 face medially. A similar arrangement is seen in all lumbar vertebrae. This type of interlocking articulation allows flexion and extension of the caudal-thoracic and lumbar region while limiting lateral movement.

The accessory process (Fig. 2-70, B) projects caudally from the pedicle ventral to the caudal articular process and over the dorsal aspect of the intervertebral foramen. It is present from the midthoracic region to the fifth or sixth lumbar vertebra. The mamillary process is a knoblike dorsal projection of the transverse processes of the second through tenth thoracic vertebrae and cranial articular processes of the eleventh thoracic through the caudal vertebrae. Epaxial muscles of the transversospinalis system attach to these processes.

Lumbar Vertebrae

The lumbar vertebrae (Fig. 2-71) have longer bodies than the thoracic vertebrae. The transverse processes are directed cranially as well as ventrolaterally. The articular processes lie mainly in sagittal planes. The caudal processes protrude between the cranial ones of succeeding vertebrae. The prominent blunt spinous processes are largest in the midlumbar region and have a slight cranial direction. The seventh lumbar vertebra is slightly shorter than the other lumbar vertebrae.

Sacrum

The sacrum (Fig. 2-72) results from the fusion of the bodies and processes of three vertebrae. This bone lies between the ilia and firmly articulates with them.

The body of the first segment is larger than the combined bodies of the other two segments. The three are united to form a concave ventral surface.

The dorsal surface presents several markings that result from the fusion of the three sacral vertebrae. The median sacral crest represents the fusion of the three spinous processes. The dorsal surface also bears two pairs of dorsal sacral foramina, which transmit the dorsal branches of the first two sacral spinal nerves.

The pelvic (ventral) surface has two pairs of pelvic sacral foramina. They transmit the ventral branches of the first two sacral spinal nerves. The wing of the sacrum is the enlarged lateral part that bears a large, rough surface, the auricular face, which articulates with the ilium.

The base of the sacrum faces cranially. The ventral part of the base has a transverse ridge, the promontory. This, with the ilia, forms the dorsal boundary of the pelvic inlet.

Caudal Vertebrae

The average number of caudal vertebrae (Fig. 2-73) in the dog is 20. These lose their distinctive features as one proceeds caudally.

Ribs

All 13 pairs of ribs (Figs. 2-74, 2-75) have dorsal bony and ventral cartilaginous parts that meet at a costochondral junction. This junction may be expanded in rapidly growing animals. The cartilaginous parts are called costal cartilages. The first nine pairs of ribs articulate directly with the sternum. The costal cartilages of the tenth, eleventh, and twelfth unite with one another to form the costal arch. The thirteenth rib often ends freely in the flank. The bony part of a typical rib presents a head, neck, tuberculum, and body.

The head of ribs 1 through 10 articulates with the costal foveae of two contiguous vertebrae and the intervening fibrocartilage. For ribs 11 through 13, the head articulates only with the cranial costal fovea on the body of the vertebra of the same number. The tubercle of the rib articulates with the costal fovea of the transverse process of the vertebra of the same number. Between the head and tuberculum of the rib is the neck.

Sternum

The sternum is composed of eight unpaired segments, the sternebrae (Figs. 2-74, 2-75). Consecutive sternebrae are joined by the intersternebral cartilages. The first sternebra, also known as the manubrium, ends cranially in a clublike enlargement. The last sternebra is flattened dorsoventrally and is called the xiphoid process. The caudal end of this process is continued by a thin plate, the xiphoid cartilage.

Hypaxial Muscles

Epaxial Muscles

The dorsal trunk musculature associated with the vertebral column and ribs may be divided into three parallel longitudinal muscle masses on each side. Each is composed of many overlapping fascicles. These three columns include the lateral iliocostalis system, the intermediate longissimus system, and the medial transversospinalis system (Fig. 2-81). Various fusions occur between these columns, giving rise to different muscle patterns that are difficult to separate. These muscles act as extensors of the vertebral column and also produce lateral movements of the trunk when contracting on only one side.

Vertebral Joints

The atlanto-occipital joint (Fig. 2-83) is continuous with the atlantoaxial joint through the articulation of the dens with the body of the atlas. The dens is held against the fovea dentis by the transverse ligament of the atlas, which passes dorsal to it and is attached to the body on both sides. Apical and alar ligaments pass from the cranial end of the dens to the basioccipital bone between the occipital condyles. The spine of the axis is joined to the arch of the atlas by a thick dorsal atlantoaxial ligament (Fig. 2-84).

The remaining vertebrae articulate by synovial joints between articular processes and by fibrous joints between the bodies. The latter are intervertebral discs (Figs. 2-85 through 2-87), which consist of outer circumferential collagenous fibers, the anulus fibrosus, and an inner gelatinous core, the nucleus pulposus. The anulus is usually thicker ventrally.

Narrow longitudinal ligaments, one within the vertebral canal and the other beneath the vertebrae, extend across all of the vertebral bodies. The ventral longitudinal ligament (Fig. 2-85) is on the ventral surface of the vertebral bodies and extends from the sacrum to the axis. It is best developed in the caudal thoracic and lumbar regions. The thicker dorsal longitudinal ligament (Fig. 2-86) is on the midline of the floor of the vertebral canal ventral to the spinal cord. It widens where it passes over and attaches to the anulus fibrosus of the intervertebral discs. It extends cranially to the axis.

Yellow ligaments extend between vertebral arches to cover the epidural interarcuate space between the articular processes. Interspinous ligaments (Fig. 2-84) connect adjacent spines above the arches. The supraspinous ligament is a longitudinal band of fibrous connective tissue that connects the apices of all spinous processes from the level of the third caudal vertebra to the first thoracic vertebra. The cranial continuation of this ligament into the cervical region is called the nuchal ligament (Fig. 2-84). The nuchal ligament is elastic and paired. In the dog it extends from the apex of the first thoracic spine to the spine of the axis.

Ribs

The head of each rib articulates in a synovial joint with the craniodorsal aspect of the vertebra of the same number. For the first 10 ribs this articulation includes the caudodorsal portion of the next cranial vertebral body. These adjacent vertebral articular surfaces are cranial and caudal costal foveae. The tuberculum of each rib except the last few has a fibrous articulation with the transverse process.

From the second through the tenth ribs, where the rib heads articulate between vertebral bodies, an intercapital ligament (Figs. 2-85, 2-86) connecting right and left rib heads extends across the dorsal aspect of the anulus fibrosus ventral to the dorsal longitudinal ligament. This intercapital ligament holds the head of the rib in its joint and may provide additional containment for the intervertebral disc, which has a lower incidence of extrusion in this region.

The sternal part of each rib is cartilaginous. The second to seventh ribs join the sternum at modified synovial joints. Other ribs join the sternum as continuous fibrocartilages.