Thirteen pairs of ribs are normally present, of which eight are sternal
and five asternal. They are in general longer, wider, flatter, less
curved, and less regular in form than in the horse. The eighth, ninth,
and tenth are the longest and widest. The width of most of the ribs
increases considerably in the middle, and the breadth of the intercostal
spaces is correspondingly diminished. The neck is long, and forms
(except in the posterior part of the series) a smaller angle with the
shaft than in the horse. The articular surface of the tubercle is
concave transversely, except on the last two or three, where the facet
is small and flat or absent. The lower ends of the second to the tenth
or eleventh inclusive form diarthrodial joints with the costal
cartilages. The first costal cartilages are very short; they articulate
by their internal surfaces with the sternum, but not with each other.
The presence of a fourteenth rib is not very rare. It is usually
floating and may correspond to an additional thoracic vertebra or to
the first lumbar. Reduction of the thirteenth is more common. The
eighth cartilage often does not reach the sternum, but articulates
with the seventh.
The =sternum= consists of seven sternebræ, most of which are developed
from two lateral centers. It is wider, flatter, and relatively longer
than in the horse, and the ventral crest or “keel” is absent. The
manubrium is somewhat wedge-shaped and laterally compressed. Its base
forms a diarthrodial joint with the body of the bone, and laterally it
bears extensive facets for articulation with the first pair of costal
cartilages. The body widens from before backward, but behind the last
pair of costal facets it becomes much narrower. The ventral surface is
prominent on the second and third segments, concave further back. The
lateral borders are notched for the passage of vessels. The cariniform
cartilage is absent. The xiphoid cartilage is like that of the horse but
is smaller.