The vertebral formula is C_{7}T_{13}L_{6}S_{5}Cy_{18–20}.
The =cervical vertebræ= are much shorter than those of the horse and are
smaller in their other dimensions. The =articular processes= are smaller
than in the horse, and a plate of bone connects each two of the same
side. The transverse processes of the third, fourth, and fifth are
double; the upper part projects backward, and is short and stout; the
lower part is directed downward and forward, and is longer and more
plate-like. The lower part of the sixth transverse process is a large,
thick, quadrilateral and almost sagittal plate, directed downward. The
seventh transverse process is single, short, and thick, and presents no
foramen transversarium; it is in series with the upper part of the
preceding processes. The =spinous processes= are well developed, and
increase in height from before backward. They are directed upward and
forward, with the exception of the last, which is nearly vertical and is
about four or five inches (ca. 10 to 12 cm.) in height. The summit of
that of the third bone is usually bifid. The =ventral spines= are
prominent and thick in their posterior part; they are absent on the last
two.
The =atlas= has a large rough =tuberosity= on its dorsal arch. The
=ventral arch= is very thick. The =wings= are less curved than in the
horse, and the foramen transversarium (posterior foramen) is absent. The
cavities for the occipital condyles are divided into upper and lower
parts by a non-articular area, and are separated by a narrow interval
below. The =posterior articular surfaces= are flattened behind and are
continued into the vertebral canal, forming an extensive area for the
odontoid process of the axis.
The =axis= is short. The =spine= projects a little in front, and
increases in height and thickness behind; its posterior border descends
abruptly. The =odontoid process= (dens) is wide, and its upper surface
is deeply concave from side to side. The =intervertebral foramen= is
circular and not so close to the anterior border of the arch as in the
horse. The =posterior notches= are not so deep. The =transverse
processes= are stouter, but the =foramen transversarium= is small and
sometimes absent.
The =thoracic vertebræ=, thirteen in number, are larger than those of
the horse. The =bodies= are longer and are distinctly constricted in the
middle. They bear a thin-edged ventral crest. The =arches=—in addition
to the usual notches, which are shallow—are perforated in the posterior
part by a foramen. The =transverse processes= are thick and strong, and
bear rounded mammillary processes (except at the posterior end of the
series); the last two, although prominent, do not always articulate with
the ribs. The =spinous processes= are long. The first is much higher
than in the horse, the next two are usually the most prominent, and
behind this there is a very gradual diminution in height. The backward
slope, slight at first, increases to the tenth; the last is vertical and
lumbar in character. The summit is usually pointed on the first, and the
thickening on those further back is less than in the horse. The width
diminishes from the fifth to the eleventh usually. Both borders of the
spines are in general thin and sharp, but the last three or four
sometimes have thick posterior margins.
[Illustration: