92 



OSTEOLOGY. 



frequently becomes developed so as to form a bridge of bone across it, converting 

 the groove into a canal through which the vertebral artery and the posterior ramus 

 of the suboccipital nerve pass a condition normally met with in many animals. 

 It is noteworthy that the grooves traversed by the highest two spinal nerves lie 

 behind the articular processes, in place of in front, as in other parts of the column. 



The ring formed by the lateral masses and the anterior and posterior arches is 

 of irregular outline. The anterior part, cut off from the rest by the transverse 

 ligament, serves for the lodgment of the dens of the epistropheus ; the larger 

 part behind corresponds to the upper part of the vertebral canal. 



Epistropheus or Second Cervical Vertebra. This is characterised by the 

 presence of the tooth-like dens (O.T. odontoid process) which projects upwards from 

 the superior surface of the body. Slightly constricted where it joins the body, the 

 dens tapers to a blunt point superiorly, on the sides of which there are surfaces for 

 the attachment of the alar ligaments. When the atlas and epistropheus are articulated 

 this process lies behind the anterior arch of the atlas, and displays on its anterior 

 surface an oval or circular facet which rests on that on the posterior surface of the 

 anterior arch of the atlas. On the posterior aspect of the neck of the dens there 

 is a shallow, groove in which lies the transverse ligament of the atlas, which 

 holds the dens in position. 



Dens Groove for transverse ligament of the atlas 



Superior articular 

 surface 



Dens 



Spine 



Foramen 

 vertebral 

 artery 



Inferior articular 

 process 



Spine 

 A 



bral artery Transverse process 



B 



Inferior articular 

 process 



FIG. 109. EPISTROPHEUS (O.T. Axis), (A) from behind and above, (B) from the left side. 



The anterior surface of the body has a raised triangular surface, which ends 

 superiorly in a ridge passing upwards to- the neck of the dens. The roots of the 

 vertebral arches are concealed above by the superior articular processes ; inferiorly, 

 they are deeply grooved. The laminae prismatic on section are thick and strong, 

 ending in a stout, broad, and bifid spinous process, the under surface of which is 

 deeply grooved, whilst its sides meet superiorly in a ridge. Placed over the 

 roots of the vertebral arches and the anterior root of the transverse processes are 

 the superior articular surfaces. These are more or less circular in shape, slightly 

 convex from before backwards, flat from side to side, and are directed upwards 

 and a little laterally. They are channelled inferiorly by the foramina trans- 

 versaria, which turn laterally beneath them. The grooves by which the second 

 cervical nerves leave the vertebral canal cross the laminae immediately behind 

 the superior articular processes. The inferior articular processes agree in form 

 and position with those of the remaining members of the series, and are placed 

 behind the inferior intervertebral notches. The transverse process is markedly 

 down-turned, and its lateral extremity is not bifid. 



The sixth cervical vertebra often displays an enlargement of the anterior 

 tubercle on the transverse process, called the carotid tubercle from the circumstance 

 that the carotid artery may be conveniently compressed against it. It is necessary 

 to add, however, that the tubercle is not always well developed. 



