36 THE SKELETON OF THE HORSE 
The dorsal arch (Arcus dorsalis) presents a median dorsal tubercle (Tuber- 
culum dorsale) and is concave ventrally. It is perforated on either side near its 
anterior margin by the intervertebral foramen (Foramen invertebrale). The an- 
terior 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, the fovea dentis, on 
which the dens or odontoid process of the axis rests. In front of this is a transverse 
rough excavation and a ridge for the attachment of the ligamentum dentis. 
The wings or al# are modified transverse processes. They are extensive 
curved plates which project ventro-laterally 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; its position can be recog- 
nized in the living animal. 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 four centers, 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 and by two 
ventro-lateral layers. These parts are usually fused at about six months. 
THE Axis 
The axis (Axis s. Epistropheus) is the longest of the vertebrae, and is character- 
ized by the presence of the dens or odontoid process, which projects from the an- 
terior part of the body. 
The anterior extremity of the body presents centrally the dens or odontoid 
Spinous process 
Dens 
Anterior articular process 
Body 
Fie. 12.—Axts or Horse, Lerr View. 
1, Arch; 2, intervertebral foramen; 3, notch; 4, foramen transversarium. 
process (Dens axis); 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 ligamentum dentis dorsally. Flanking this on either side are the modified 
anterior articular processes, which have saddle-shaped articular surfaces confluent 
