This vertebra is decidedly atypical in form and structure. The body and
spinous process are absent. It has the form of a strong ring, from which
two curved plates, the =wings=, project laterally. The ring incloses a
very large vertebral foramen, and consists of two =lateral masses=
connected by =dorsal= and =ventral arches=.
The =lateral masses= (Massæ laterales) present two deep oval =anterior
articular cavities= (Foveæ articulares craniales) which receive the
occipital condyles; they are separated by a wide notch above and a
narrow one below. The outer margin is also notched, and a triangular
non-articular depression cuts into the inner part of each cavity. The
=posterior articular surfaces= (Facies articulares caudales) are
somewhat saddle-shaped; they are confluent on the ventral arch below,
but are widely separated above, and do not conform in shape to the
corresponding surfaces of the axis.
The =dorsal arch= (Arcus dorsalis) presents a median =dorsal tubercle=
(Tuberculum dorsale) and is concave below. It is perforated on either
side near its anterior margin by the =intervertebral foramen=. The
=anterior border= is deeply notched, and the =posterior= is thin and
concave.
The =ventral arch= (Arcus ventralis) is thicker, narrower, and less
curved than the dorsal. On its lower surface is the =ventral tubercle=
(Tuberculum ventrale), into which the terminal tendon of the longus
colli muscle is inserted. The upper face has posteriorly a transversely
concave articular surface (Fovea dentis), on which the dens or odontoid
process of the axis rests. In front of this is a transverse rough
excavation for the attachment of the odontoid ligament.
The =wings= (Alæ) are modified transverse processes. They are extensive
curved plates which project outward, downward, and backward from the
lateral masses. The dorsal surface is concave. Between the ventral
aspect of the wing and the lateral mass is a cavity, the =fossa
atlantis=; in this there is a foramen which opens into the vertebral
canal. The =border= is thick and rough. Two foramina perforate each
wing. The anterior one, the =foramen alare=, is connected with the
intervertebral foramen by a short groove. The posterior one is the
=foramen transversarium=.
=Development.=—The atlas ossifies from =three= or =four centers=, one or
two for the ventral arch, and one on either side for each lateral mass,
wing, and half of the dorsal arch. At birth the bone consists of three
pieces—the ventral arch and two lateral parts, which are separated by a
layer of cartilage in the dorsal median line. These parts are usually
fused at about six months.
[Illustration:
FIG. 11.—FIRST THREE CERVICAL VERTEBRÆ OF HORSE, LATERAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]