310 THE VERTEBRATE SKELETON. 



the odontoid process. The sides of the atlas are pierced by 

 the vertebra rterial canals, above which there are two slight 

 backwardly-projecting outgrowths bearing the postzygapophyses 

 on their inner faces. 



THE THORACIC VERTEBRAE. 



The thoracic region includes all the vertebrae bearing free 

 ribs, except the first two, viz. those whose ribs do not reach 

 the sternum. There are seven thoracic vertebrae. The first 

 four have centra with saddle-shaped articulating surfaces, but 

 are more or less firmly united together by their neural spines ; 

 the last two are completely ankylosed by their centra to the 

 lumbar vertebrae. 



Each of the first five vertebrae has a prominent, vertical, 

 abruptly terminated neural spine, and straight transverse 

 processes. The zygapophyses and articulating surfaces at the 

 ends of the centra are well developed. The third, fourth, fifth, 

 and sixth vertebrae have verv prominent hypapophyses. The 

 articular facets for the ribs are well marked, those for the tu- 

 bercula lying at the free ends of the transverse processes, and 

 those for the capitula at the sides of the anterior ends of the 

 centra. The sixth and seventh thoracic vertebrae are firmly 

 fused by their centra and neural arches to one another and 

 to the lumbar vertebrae behind, and by their transverse pro- 

 cesses to the ilia. The sixth has its centrum terminated in 

 front by a saddle-shaped articulating surface, and bears a pair 

 of prominent prezygapophyses. Its transverse processes and 

 centrum bear facets for the tubercula and capitula of the ribs 

 respectively. In the seventh vertebra the tubercular facet is 

 wanting. 



THE SACRUM. 



The sacrum generally consists of seventeen vertebrae 

 fused with one another and with the ilia. Their number may 

 be reckoned from the number of foramina for the exit of spinal 

 nerves. The two most anterior of these vertebrae bear ribs 



