3 o CERVICAL VERTEBRA*:. [chap. 



united to both. On the other hand, the atlas at the time 

 of birth has nothing corresponding to the centrum of other 

 vertebrae, its inferior arch being still cartilaginous. 



It is therefore a generally received opinion among ana- 

 tomists that the anterior ossification of the axis is essentially 

 the body of the atlas, which unites with both arch and 

 centrum of the vertebra behind it. It must be observed, 

 however, that in its mode of ossification, at least in man, it 

 differs from the centra of all the other vertebrae, as at one 

 period it consists of two distinct lateral pieces, which after 

 a while coalesce in the middle line. The usual disk-like 

 epiphyses of the vertebral bodies are represented by one 

 at the posterior extremity of the body, by a small osseous 

 nodule which completes the odontoid process in front, and 

 by some irregular ossifications found between the two main 

 portions of which the body is composed. 



The inferior arch of the atlas ossifies soon after birth 

 from one or more centres, and the resulting piece of bone 

 (Fig. 9, id) ultimately unites with the two pieces forming 

 the neural arch about the same time as that at which they 

 join together in the middle line above. This piece may^ 

 probably be regarded as a detached "hypapophysial" seg-, 

 ment of the first vertebral centrum, the remainder of which 

 forms the odontoid portion of the body of the axis. 



The cervical vertebrae of the other Primates resemble 

 those of man generally, the most noticeable deviations being 

 the following : — 



In the atlas the groove for the first cervical nerve is usually 

 i converted into a foramen; and a median hypapophysial 

 tubercle or spine often projects backwards from its inferior 

 arch under the axis (especially in Mycetes and Lagothrix). 



The spinous processes, especially of the third, fourth,! 

 fifth, and sixth cervical vertebrae, are immensely elongated 



