xi VERTEBRAL COLUMN 499 



anteriorly into a conical odontoid process, which fits into the 

 lower part of the ring of the atlas and is held in its place by 

 a ligament extending transversely across the latter : it is 

 ossified from a distinct centre, which really belongs to the 

 centrum of the atlas. The neural spine of the axis is elon- 

 gated and compressed, and its transverse processes small 

 and perforated each by a vertebrarterial canal ; zygapo- 

 pliyses are present only on the posterior face of the arch. 

 In all the other cervical vertebras, the transverse processes 

 are also perforated by vertebrarterial canals, and, except in 

 the seventh or last, are divided into dorsal and ventral 

 lame! Lie. The zygapophyses resemble those of the thoracic 

 vertebra described above. The seventh cervical vertebra 

 has a longer spine than the others, and bears a pair of half 

 facets on the posterior surface of its centrum with which the 

 first pair of ribs in part articulate. 



A typical thoracic vertebra has already been described. 

 In the tenth, the neural spine is vertical; and in the remain- 

 ing two or three; which are larger than the others, it slopes 

 forwards. In the posterior three or four there are no tuber- 

 cular facets, the ribs in this region not being forked ; the 

 capitular facets are entire, and are situated on the corre- 

 sponding centrum only. Additional processes are present 

 above the pre -zygapophyses from the ninth thoracic 

 vertebra onwards. 



The lumbar vertebras are relatively large, increasing in 

 size from before backwards, and their various processes are 

 greatly developed. The neural spines are directed upwards 

 and forwards, the transverse processes are large and pro- 

 ject outwards, downwards and forwards. As in the pos- 

 terior thoracic vertebras, there are stout processes above the 

 pre-zygapophyses (which face inwards], and there is also a 

 pair of more slender processes below the post-zygapophyses 

 (which face outwards], and a median ventral process pro- 

 jecting downwards from the centrum is present on the 

 first two. 



The sacral vertebras are fused to form the sacrum, which 

 supports the pelvic arch. The first and to a less extent 

 the second also has large, expanded, transverse processes 

 which articulate with the ilia ; these are the sacral vertebras 

 proper ; the others, which decrease in size from before back- 

 wards, are really the anterior caudal vertebras which fuse 

 with the true sacral vertebras to form a compound sacrum. 



The more anterior caudal vertebras resemble those of the 

 sacral region, but on passing backwards all the processes are 

 seen to diminish in size, until only the centra are left at the 

 end of the tail. 



