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.
