36 THE SKELETON OP THE HOESE 



, 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 bj' 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 '\\'hich 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 alae 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 mng 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 Aving. 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 tour 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 



Posterior articular 



process 



Dens • 



Anterior articular process 



Transverse process 



Fig. 12. — Axis of Horse, Left View. 

 1, Arch; S, 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 



