Bone shape

Bones can be categorised according to their shape:

Fig. 3.2 Structure of a typical long bone.

Some specialised types of bone are:

Development of bone

The process by which bone is formed is called ossification and there are two types: intramembranous and endochondral ossification. The cells responsible for laying down new bone are called osteoblasts; the cells that destroy or remodel bone are called osteoclasts.

Endochondral ossification

The process of endochondral ossification (Fig. 3.3) is as follows:

1 A cartilage model develops within the embryo.

2 Primary centres of ossification appear in the diaphysis or shaft of the bone. The cartilage is replaced as the osteoblasts lay down bone, which gradually extends towards the ends of the bone.

3 Secondary centres of ossification appear in the epiphyses or ends of the bone, continuing the bone development.

4 Osteoclasts then start to remove bone from the centre of the diaphysis to form the marrow cavity, while the osteoblasts continue to lay down bone in the outer edges.

5 Between the diaphysis and epiphyses a narrow band of cartilage persists. This is the growth plate or epiphyseal plate, which allows the bone to lengthen while the animal is growing. Eventually, when the animal has reached its final size, this will be replaced by bone and growth will no longer be possible. The epiphyseal plate is then said to have ‘closed’ and the time at which it happens is different for each type of bone.

Fig. 3.3 Stages of endochondral ossification. A A cartilage model of the bone exists in the embryo. B Ossification begins from the primary ossification centre in the shaft (diaphysis). C Secondary ossification centres appear in the ends of the bone (epiphyses). D Ossification continues in primary and secondary centres; osteoclasts start to break down bone in the shaft, creating the marrow cavity. E The first growth plate fuses and growth is now only possible at the proximal growth plate; the medullary cavity extends into the epiphysis. F The proximal growth plate fuses and bone growth ceases.

Rickets is a disease of young growing animals caused by a nutritional deficiency of vitamin D or phosphorus. The bones fail to calcify and become bowed, and the joints appear swollen because of enlargement of the epiphyses. Any animal kept permanently inside is at risk of developing rickets because vitamin D is formed by the action of ultraviolet light on the skin.

The skeleton

The skeleton (Fig. 3.1) can be divided into three parts.

When studying the anatomy of the skeletal system it is helpful to understand the terms that are used to describe the various projections, passages and depressions that are found on and within bones.

The skull

The bones of the head include the skull, nasal chambers, mandible or lower jaw and hyoid apparatus. The functions of the skull are:

Cranium

The caudal part of the skull that provides the bony ‘case’ in which the brain sits is called the cranium (Figs 3.4, 3.5). The bones of the cranium include:

• Parietal – forms much of the dorsal and lateral walls of the cranium

• Temporal – lies below the parietal bone on the caudolateral surface of the skull. The most ventral part of the temporal bone forms a rounded prominence called the tympanic bulla, which houses the structures of the middle ear. There is an opening into the tympanic bulla, called the external acoustic meatus, which in life is closed by the tympanic membrane or eardrum. The cartilages of the external ear canal are attached to this region

• Frontal – forms the front aspect of the cranium or ‘forehead’. Contains an air-filled chamber called the frontal sinus, which connects to the nasal chamber

• Occipital – this lies at the base of the skull on the caudal aspect. In this region there is a large hole called the foramen magnum, through which the spinal cord passes. On either side is a pair of bony prominences, the occipital condyles. These articulate with the first cervical vertebra or atlas (Fig. 3.1). At the side of the occipital condyles are the jugular processes, which are sites for muscle attachment

• Sphenoid – this lies on the ventral aspect of the skull, forming the floor of the cranial cavity. It is penetrated by many small foramina through which nerves and blood vessels pass

• Sagittal crest – a ridge of bone on the dorsal midline surface of the skull, which can be prominent in muscular dogs

• Zygomatic – the zygomatic arch is an arch of bone that projects laterally from the skull, forming the ‘cheekbone’

• Lacrimal – lies at the base of the orbit, which houses the eye, and is the region through which the tears drain from the eye into the nose.

Fig. 3.4 Lateral view of the dog skull showing main bony features.

Fig. 3.5 Ventral view of the dog skull showing main bony features.

Mandible

The mandible or lower jaw is comprised of two halves or dentaries, joined together at the chin by a cartilaginous joint called the mandibular symphysis. Each half is divided into a horizontal part, the body, and a vertical part, the ramus (Fig. 3.6). The body carries the sockets or alveoli for the teeth of the lower jaw. The ramus articulates with the rest of the skull at the temporomandibular joint via a projection called the condylar process. A rounded coronoid process, which projects from the ramus into the temporal fossa, is the point to which the temporalis muscle attaches (Fig. 3.6). There is a depression on the lateral surface of the ramus, the masseteric fossa, in which the masseter muscle lies.

Fig. 3.6 Lateral (A) and medial (B) views of the dog mandible.

Hyoid apparatus

The hyoid apparatus lies in the intermandibular space and consists of a number of fine bones and cartilages joined together in an arrangement that resembles a trapeze (Fig. 3.7). The hyoid apparatus is the means by which the larynx and tongue are suspended from the skull. The apparatus articulates with the temporal region of the skull in a cartilaginous joint.

Fig. 3.7 The trapeze-like hyoid apparatus suspended from the skull (dog).

Skull shapes

The shape of the skull varies between species. In the domestic cat the skull is much more rounded or ‘apple-shaped’ than it is in the dog, and there is little difference between the various cat breeds. In the dog, although the basic anatomy remains the same, the overall appearance differs greatly between the different breeds (Fig. 3.8).

Fig. 3.8 The three basic shapes of the dog skull seen in common breeds. Also shows the positions of the maxillary and frontal sinuses.

Three morphological forms of dog skull are recognised:

The shape of the skull, particularly in brachycephalic breeds such as the bulldog, can lead to a variety of clinical conditions, e.g. snoring respiration, difficulty in breathing. The relative size of the head can also cause difficulties with whelping. Breeds with a mesaticephalic shape of skull are much less likely to exhibit these conditions.

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• Dolichocephalic – the head, particularly the nose, is long and narrow, e.g. Greyhound, Borzoi, Afghan hound

• Mesaticephalic – (mes meaning ‘middle’) is the ‘normal’ or average shape of the dog skull, e.g. Beagle, Labrador, Pointer

• Brachycephalic – the cranium is often more rounded and the nose is short and may be pushed in, because of shortening of the nasal chambers, hard palate and mandible, e.g. Bulldog, Pekinese, Boxer, Pug.

The vertebrae

The vertebral column is comprised of a number of bones arranged in a series along the midline of the body and extending from the base of the skull to the tip of the tail. The vertebrae are divided into regions depending upon their position in the body:

Each species has a characteristic number of vertebrae within each region, which is written as a formula. In the dog and cat this formula is: C7, T13, L7, S3, Cd20–23 (Fig. 3.1).

The functions of the vertebral column are:

Basic plan of a vertebra

The vertebrae within all regions of the vertebral column are basically similar in structure, although each region shows slight differences related to function. A typical vertebra (Fig. 3.9) consists of a roughly cylindrical ventral body with a convex cranial end and a concave caudal end. This arrangement enables the bodies to fit together, creating a flexible rod. The body is topped by an arch, called the vertebral or neural arch, which forms a tunnel-like vertebral foramen through which the spinal cord passes. When linked together, the vertebral foramina constitute the spinal canal.

Fig. 3.9 Basic structure of a typical vertebra.

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The neural arch has a dorsal projection called the spinous process or neural spine, which varies in height and size from one region of the vertebral column to another. On either side of the vertebra there are laterally projecting processes, the transverse processes, which also vary in shape and size between regions. The transverse processes divide the muscles of the vertebral column into dorsal or epaxial and ventral or hypaxial divisions (see muscles of the trunk, p 48).

On the cranial and caudal edges of the arch of each vertebra there are cranial and caudal articular processes. These form a synovial joint with those of the adjacent vertebrae, creating a flexible rod running in the midline of the body. There are also a number of other processes, which are sites for muscle attachment. Lying between the bodies of each pair of vertebrae is a fibrocartilaginous intervertebral disc, which acts as a ‘shock absorber’, preventing damage to the spinal cord (Fig. 3.10). The intervertebral disc is composed of a tough fibrous connective tissue outer area, called the annulus fibrosus, and a core of gelatinous material called the nucleus pulposus.

Fig. 3.10 The intervertebral disc, positioned between vertebrae and acting as a ‘cushion’.

The vertebrae are linked by ligaments that run between them and they articulate with one another by two types of joint:

• Cartilaginous – between the bodies of each vertebra

• Synovial – between the cranial and caudal articular processes.

In the case of a ‘slipped disc’, the annulus fibrosus ruptures and the pulpy centre of the disc protrudes outward. This puts pressure either on the spinal cord or on the associated nerves leaving the cord, causing the animal to show symptoms ranging from pain to paralysis due to the loss of motor function.

Regional variations

• Cervical vertebrae (Fig. 3.11A). There are always seven cervical vertebrae in the neck of all mammals. The first cervical vertebra or atlas has a unique and distinctive shape. The atlas does not have a body or a spinous process, but consists of two large wing-like lateral masses joined by a ventral and dorsal arch. The second cervical vertebra or axis is also unusual and has an elongated, blade-like spinous process, which serves as a point of attachment for neck muscles. A strong ligament, called the nuchal ligament, also attaches to the spinous process and extends from the axis to the first thoracic vertebra. On the cranial aspect of the axis, a projection of bone called the dens or odontoid process fits into the vertebral foramen of the atlas and serves as a pivot around which the atlas can be rotated. The remaining cervical vertebrae (C3–C7) follow the basic vertebral plan, and get progressively smaller as they advance towards the junction with the thoracic vertebrae.

• Thoracic vertebrae (Fig. 3.11B). There are usually 13 thoracic vertebrae. Their distinguishing feature is their tall spinous processes and short bodies. They articulate with the ribs at two sites: the costal fovea, which forms a synovial joint with the head of the rib, and the transverse fovea, which forms a synovial joint with the tubercle of the rib. The height of the spinous processes decreases as the series progresses towards the lumbar region.

• Lumbar vertebrae (Fig. 3.11C). There are usually seven lumbar vertebrae. These vertebrae have large bodies and long transverse processes angled cranioventrally, to which the lumbar muscles attach.

Fig. 3.11 Variations in the shape of vertebrae. A Cervical. B Thoracic. C Lumbar. D Sacral (the three sacral vertebrae are fused into one bone). E Caudal/coccygeal.

The synovial joint between the atlas (C1) and the occipital condyles of the skull allows nodding movements of the head, and the synovial joint between the atlas and axis (C1–C2) allows a pivotal movement so that the head can turn in all directions.

• Sacral vertebrae (Fig. 3.11D). These three vertebrae are fused together to form the sacrum in the adult dog and cat. The sacrum forms a fibrosynovial joint with the wing of the ilium of the pelvic girdle: the sacroiliac joint.

• Caudal or coccygeal vertebrae (Fig. 3.11E). These vary in number and shape according to the length of the tail. The first few resemble the lumbar vertebrae, but they get progressively smaller and simpler throughout the series. The last few caudal vertebrae are reduced to little rods of bone.

The ribs and sternum

The ribs form the walls of the bony thoracic cage that protects the organs of the chest. There are 13 pairs of ribs in the dog and cat, which articulate with the thoracic vertebrae (Fig. 3.12). A rib is a flat bone consisting of compact bone on the outside packed with cancellous bone on the inside. Each rib has a bony dorsal part and a cartilaginous ventral part – the costal cartilage. The most dorsal part of the bony rib has two projections: the head, which articulates with the costal fovea of the vertebra, and the tubercle or neck, which articulates with the transverse fovea of the appropriate thoracic vertebra.

Fig. 3.12 Structure of the canine rib. A Caudal view. B Lateral view showing articulation with a thoracic vertebra.

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 112.)

The costal cartilage articulates with the sternum, either directly or indirectly. The first eight pairs of ribs attach directly to the sternum and are called the sternal ribs. The ribs from pairs 9–12 are called asternal or ‘false’ ribs, and they attach via their costal cartilages to the adjacent rib, forming the costal arch. The last ribs (pair 13) have no attachment at their cartilaginous ends, which lie free in the abdominal muscle – this pair are called the ‘floating’ ribs. The space between each successive pair of ribs is called the intercostal space and is filled by the intercostal muscles of the trunk (Fig. 3.13).

Fig. 3.13 The rib cage of the dog, showing sternal, asternal and ‘floating’ ribs.

(With permission from Colville T, Bassett JM 2001 Clinical anatomy and physiology for veterinary technicians. Mosby, St Louis, MO, p 112.)

The sternum forms the floor of the thoracic cage (Fig. 3.13) and is composed of eight bones, the sternebrae, and the intersternebral cartilages. The most cranial sternebra is the manubrium, which projects in front of the first pair of ribs and forms part of the cranial thoracic inlet. Sternebrae 2–7 are short cylindrical bones. The last sternebra is longer and dorsoventrally flattened and is called the xiphoid process. Attached to the xiphoid process and projecting caudally is a flap of cartilage called the xiphoid cartilage. The linea alba attaches to this. Between each pair of sternebrae are cartilaginous discs called the intersternebral cartilages.

Bones of the forelimb

The bones of the forelimb (Fig. 3.1) are:

Fig. 3.14 The dog scapula. A Lateral view. B Ventral view.

Fig. 3.15 The dog humerus. (Cats do not have a supratrochlear foramen.)

Fig. 3.16 A The dog radius. B The dog ulna (cranial and lateral views).

Fig. 3.17 Lateral and craniocaudal views of the canine elbow.

(Courtesy of Panagiotis Mantis.)

Fig. 3.18 A Bones of the distal forelimb in the dog. CI, CII, etc. = carpal bones 1, 2, etc. MC = metacarpal bones. Metacarpal bone 1 is the dew claw. B Lateral and dorsopalmar views of the canine carpus and distal forelimb.

(Courtesy of Panagiotis Mantis.)

Bones of the hindlimb

The bones of the hindlimb (Fig. 3.1) are:

Fig. 3.19 A Bones and bony features of the dog pelvis. B Ventrodorsal view of the canine pelvis.

(Courtesy of Richard Aspinall.)

Fig. 3.20 The dog femur.

Fig. 3.21 Lateral view of the canine stifle joint.

(Courtesy of Richard Aspinall.)

Fig. 3.22 The dog tibia and fibula.

Fig. 3.23 A The dog tarsus or hindpaw. TI, TII, etc. = tarsal bones 1, 2, etc. MT = metatarsal bones. B Lateral and dorsoplantar views of the canine tarsus and distal hindlimb.

(Courtesy of Panagiotis Mantis.)

Fig. 3.25 A The elbow joint, lateral view. B The tarsus or hock joint, lateral view. C The stifle joint, cranial and medial views.

Splanchnic skeleton

This is composed of the splanchnic bones. A splanchnic bone is a bone that develops in soft tissue and is unattached to the rest of the skeleton. The only example of a splanchnic bone in the dog and cat is the bone of the penis, the os penis. The urethra lies in the urethral groove, on the ventral surface of the os penis in the dog. In the cat the urethral groove is on the dorsal surface of the os penis, because of the different orientation of the penis (see Ch. 11).

The cow has a splanchnic bone in its heart, called the os cordis, while birds have splanchnic bones forming a rim around the eye to provide strength to the large eyeball (see Ch. 13).

Fibrous joints

Fibrous joints are immovable joints and the bones forming them are united by dense fibrous connective tissue, e.g. in the skull fibrous joints unite the majority of the component bones and are called sutures. The teeth are attached to the bony sockets in the jaw bone by fibrous joints.

Fibrous joints are also classed as synarthroses, i.e. a type of joint that permits little or no movement. Some cartilaginous joints also fall into this category.

Cartilaginous joints

Cartilaginous joints allow limited movement or no movement at all and are united by cartilage, e.g. the pubic symphysis connecting the two hip bones and the mandibular symphysis joining the two halves of the mandible. Both these joints are also classed as synarthroses.

Some cartilaginous joint may also be classed as amphiarthroses, which allow some degree of movement between the bones, e.g. between the bodies of the vertebrae, allowing for limited flexibility of the spinal column.

Synovial joints

Synovial joints or diarthroses allow a wide range of movement. In synovial joints, the bones are separated by a space filled with synovial fluid known as the joint cavity (Fig. 3.24). A joint capsule surrounds the whole joint; the outer layer consists of fibrous tissue, which serves as protection, and the joint cavity is lined by the synovial membrane, which secretes synovial fluid. This lubricates the joint and provides nutrition for the hyaline articular cartilage covering the ends of the bone. Synovial fluid is a straw-coloured viscous fluid that may be present in quite large quantities in large joints, especially in animals that have a lot of exercise.

Fig. 3.24 Structure of a typical synovial joint.

Some synovial joints may have additional stabilisation from thickened ligaments within the fibres of the joint capsule. These are most commonly found on either side of the joint, where they are called collateral ligaments. However, other synovial joints have stabilising ligaments attached to the articulating bones within the joint – these are known as intracapsular ligaments and examples include the cruciate ligaments within the stifle joint (Fig. 3.25).

A few synovial joints possess one or more intraarticular fibrocartilaginous discs or menisci within the joint cavity. These are found in the stifle joint – it has two crescent shaped menisci – and in the temporomandibular joint between the mandible and the skull. These structures help to increase the range of movement of the joint and act as ‘shock absorbers’, reducing wear and tear.

Synovial joints allow considerable freedom of movement between the articulating bones, the extent of which depends upon the type of synovial joint. The movement allowed by a synovial joint may be in a single plane only, or in multiple planes.

Synovial joints can be further classified into subcategories based upon the types of movement that they allow (Table 3.1).

Table 3.1 Properties of different synovial joint types