Components of the skeleton

Components of the skeleton

(i) Vertebral Column (See Figs. 1-N and 1-O)

The vertebral column consists of a series of unpaired, median irregular bones, the

vertebrae, extending from the skull to the tail. Some vertebrae may become

fused (e.g. in the sacral region). The number of vertebrae in each body region

(cervical, thoracic, lumbar sacral, caudal) is fairly constant so a vertebral

formula can be expressed (e.g. C7T18L6S5Cd15-21 for the horse, C7T13L7S3Cd20-23 for

the dog). Note the differences from the human vertebral formula, particularly in

the number of thoracic vertebrae (and hence ribs). Note also the differences

between the two species, again especially in the thoracic region.

A typical vertebra consists of:

(1) BODY - cylindrical mass articulating cranially (convex) and caudally

(concave) with other vertebrae.

(2) ARCH - two lateral halves which, together with the body, form the vertebral

foramen containing the spinal cord and its vessels. The base of the

arch (pedicel) has vertebral notches for the passage of spinal nerves and

vessels.

(3) PROCESSES

articular processes, two anterior and two posterior articulate with adjacent

vertebrae.

spinous process projects from the middle of the arch for the attachment of

muscles and ligaments

transverse process, two project laterally from the arch.

mammillary process, in most mammals on the posterior thoracic and

anterior lumbar vertebrae, between the transverse and anterior articular

process.

Vertebrae from different regions are distinguished by characteristic features:

Cervical – transverse foramina (except C7), atlas and axis unique

Throacic – long, caudally pointing spinous processes until T11 (anticlinal);

Costal fovae for ribs

Lumbar – long cranially pointing transverse processes

Sacral – fusion, processes become crests, notches for nerves become foramina

Caudal – reduction in processes

Figure 1-N: Cervical vertebrae of the dog

Figure 1-O: Vertebrae of the dog (Thoracic, lumbar, sacral and caudal

vertebrae)

(ii) Ribs and Sternum (See Fig. 1-P)

Ribs are paired curves bones which form the skeleton of the thoracic wall. The

shaft of the rib varies in length, width and curvature. The costal groove on the

medial surface contains the intercostal vein. The head is at the end of the rib and

articulates with two adjacent vertebrae. It is connected by a neck to the shaft and

the tubercle which is directed caudally, articulating with the transverse process of

the vertebra.

The sternal (ventral) end of the ribs is usually slightly enlarged and roughened

where it joins the costal cartilage.

Three types of ribs can be distinguished:

(1) STERNAL RIBS - articulate with the sternum by the costal cartilage.

(2) ASTERNAL RIBS - do not articulate with the sternum, but may have

overlapping costal cartilages, united by elastic tissue to form a costal arch.

(3) FLOATING RIBS - ventral ends free and not attached to adjacent ribs.

The sternum is composed of 6-8 unpaired median bony segments (sternebrae).

Its shape varies considerably. The cranial end (manubrium) is relatively small

and terminates in cartilage (cariniform) providing attachment for breast and neck

muscles. The body of the sternum articulates with the costal cartilages of sternal

ribs and gives attachment to pectoral muscles. The ventral border of the sternum

may have a prominent crest. The sternum terminates caudally in a thin plate of

xiphoid cartilage which provides attachment for the diaphragm and abdominal

muscles.

(iii) Thoracic Limb (See Fig. 1-Q)

The shoulder (pectoral) girdle typically consists of three parts, (scapula,

coracoid and clavicle):

• The scapula is the only well developed component in domesticated mammals.

It is a flat bone, with its lateral surface divided by a spine. This

terminates in the acromion in the dog. The glenoid cavity articulates with

the head of the humerus and the supraglenoid tubercle provides attachment

for biceps brachii muscles.

• The coracoid process has fused with the scapula

• The clavicle is either absent (horse) or rudimentary in the

brachiocephalicus muscle (dog). Thus, the shoulder has no articulation

with the axial skeleton, but is supported by a syndesmosis (see chapter 3).

Humerus:

A long bone, with a cylindrical, slightly twisted shaft. The anterior surface of the

shaft has a deltoid tuberosity. The proximal end has a head (which articulates

with the glenoid cavity) a neck and greater and lesser tubercles. The distal

extremity consists of an oblique trochlear surface for articulation with the radius

and ulna, and medial and lateral epicondyles separated caudally by the olecranon

fossa and cranial by the radial (coronoid) fossa. The radial and olecranon

fossae are often joined by a large supratrochlear foramen.