The =axis= (Epistropheus) is the longest of the vertebræ, and is
characterized by the presence of the odontoid process, which projects
from the anterior part of the body.
The =body= has a median ventral crest which terminates in a tubercle
behind. The anterior extremity presents centrally the =odontoid process=
(Dens); this has a convex articular surface ventrally for articulation
with the ventral arch of the atlas, and two rough depressions for the
attachment of the odontoid ligament dorsally. Flanking this on either
side are the modified =anterior articular processes=, which have
saddle-shaped articular surfaces confluent below with that of the dens.
The posterior extremity has the usual cavity.
The =arch= presents in the young subject a notch on each side of its
anterior border; this is converted into a foramen by a ligament which
usually ossifies later. The posterior border has the usual notches.
The =posterior articular processes= are typical.
The =transverse processes= are small, single, and project backward. The
=foramen transversarium= is small.
The =spinous process= is very large and strong. Its free border is
rough, thickens posteriorly, and is continued to the articular processes
by two ridges. The lateral surfaces are concave and rough for muscular
attachment.
=Development.=—The axis has =six= or =seven centers= of ossification. In
addition to the usual five, one or two appear for the dens, which is
regarded as the displaced body of the atlas. A nucleus behind the dens,
which remains distinct to three or four years of age, is considered by
Lesbre to be the head of the axis.
[Illustration:
FIG. 12.—OCCIPITAL BONE AND FIRST THREE CERVICAL VERTEBRÆ OF HORSE,
DORSAL VIEW. (After Schmaltz, Atlas d. Anat. d. Pferdes.)
]