258 ESSENTIALS OF ZOOLOGY 



articular surfaces are borne on two pairs of articular processes 

 (zygapophyses), anterior and posterior. As in the frog, the 

 posterior processes overlap the anterior, and this holds good 

 universally. 



The thoracic vertebrae are different in that the transverse 

 processes are short, and all provide articulation for the ribs. 

 It is for this reason that they are called costal vertebrae. The 

 head of the rib is typically articulated between two centra, 

 each of which therefore bears a half-facet, and the tubercle of 

 the rib to the transverse process of the posterior of the two 

 vertebrae concerned. The spines gradually increase in length 

 from the posterior end of the series, and reach a culmination 

 in the region of the shoulders or withers. In front of this 

 region the spines decrease rapidly in length. 



The cervical vertebrae are at once distinguished by their 

 large articular processes and facets, by the vertebrarterial 

 canal, the almost flat spines, and by their relatively large size. 

 The vertebrarterial canal transmits the vertebral artery and is 

 formed by the transverse process above and a rudimentary rib 

 below ; the combined process is described as the transverse 

 process. The first two vertebrae are modified to provide 

 rotatory movements of the head. The first, or atlas, is firmly 

 articulated to the skull. It is in the form of a ring which 

 projects widely on each side into transverse processes. It 

 bears on the skull aspect a pair of articular surfaces which are 

 applied to the occipital condyles, and posteriorly a pair of 

 surfaces for articulation with the second vertebra. The ring 

 is crossed transversely by a ligament which divides it into a 

 ventral portion into which the odontoid process of the second 

 vertebra projects, and a dorsal portion which transmits the 

 spinal cord. The axis, or second vertebra, is like its successors 

 posteriorly, but anteriorly the centrum is produced into the 

 odontoid peg, a derivative of the centrum of the first vertebra, 

 which becomes fused with the centrum of the second vertebra. 

 Anterior articular processes are absent. The odontoid process, 

 or dens, is connected by ligaments both with the atlas and the 

 occipital. 



The sacral vertebrae are fused into a compact structure 

 which serves to complete the arch formed by the two innominate 



