xi VERTEBRAL COLUMN 499 



axis, has its centrum produced 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 ossi- 

 fied from a distinct centre, which really belongs to the centrum O A 

 the atlas. The neural spine of the axis is elongated and compressed, 

 and its transverse processes small and perforated each by a vertebrarterial 

 canal. Zygapophyses are present only on the posterior face of the 

 arch. In all the other cervical vertebrae, the transverse processes are 

 also perforated by the vertebrarterial canal, and except in the seventh 

 or last, are divided into dorsal and ventral lamellae. 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 remaining two or three, which are 

 larger than the others, it slopes forwards. In the posterior three or 

 four there are no tubercular facets, the ribs in this region not being 

 forked ; the capitular facets are entire, and are situated on the 

 corresponding centrum only. Additional processes are present above 

 the pre-zygapophyses from the ninth thoracic vertebra onwards. 



The lumbar vertebrae are relatively large, increasing in size from 

 before backwards, and their various processes are greatly de- 

 veloped. The neural spines are directed upwards and forwards, the 

 transverse processes are large and project outwards, downwards, and for- 

 wards. As in the posterior thoracic vertebrae, there are stout processes 

 above the pre-zygapophyses, and there is also a pair of more slender 

 processes below the post -zygapophyses and a median ventral process 

 projecting downwards from the centrum in the first two. 



The sacral vertebrae are fused together to form the sacrum, which sup- 

 ports 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 vertebrae proper, the others, which decrease in size 

 from before backwards, are really the anterior caudal vertebrae which 

 fuse with the true sacral vertebrae to form a compound sacrum. 



The more anterior caudal vertebrae resemble those of the sacral 

 region, but on passing backwards all the processes are seen to diminish 

 in size, until nothing but the centra are left at the end of the 

 tail. 



K K 2 



