84 COMPARATIVE ANATOMY 



The different shaped ends of the centra which have already been 

 mentioned are brought about after the vertebrae are quite definitely 

 formed. The centra with their arches are in a quite definite position 

 and the centra cannot therefore grow any more except at the ends. 

 These may have more substance laid down in the intervertebral regions, 

 however, and thus ultimately come to be amphicoelous. procoelous, 

 opisthocoelous, or amphiplatyan. 



REGIONS OF THE VERTEBRAL COLUMN 



The regions of the spinal column are : 



(1) Cervical. The neck region, either without ribs of any kind or 

 the ribs are smaller than in the other regions. 



(2) Thoracic. These have distinct ribs attached. 



(3) Lumbar. Following the thoracic, and without ribs. 



(4) Sacral. This region includes one or more vertebrae with which 

 the pelvic girdle is connected. 



(5) Caudal. The tail-portion immediately following the sacrum. 

 These divisions are quite distinct in the higher vertebrates, but in 



;he lower, any and all combinations may form, so that the ribs may ex- 

 tend almost the entire length of the spinal column. In such cases all 

 vertebrae having ribs are called dorsal. 



In fishes, snakes, and whales, the sacral region cannot be distin- 

 guished, and in the fishes the dorsal and cervical vertebrae are quite 

 indistinguishable. In this latter case there are, therefore, only trunk or 

 abdominal vertebrae, and caudal vertebra, the line being drawn where 

 the haemal arches begin to have ribs attached. 



The first cervical vertebra to which the skull is articulated is called 

 the atlas in all higher vertebrata, while the second cervical vertebra, at 

 least in the amniotes, is called the axis or epistropheus (Fig. 404). 



In mammals the atlas can always be distinguished by the two an- 

 terior articulating surfaces for the two condyles of the skull, and the 

 axis, by the tooth-like projection known as the odontoid process, on 

 which the atlas turns. 



It is interesting to note that embryologically, this tooth-like process 

 develops from the atlas, but then separates and later becomes attached 

 to the next succeeding vertebra. 



In a few reptiles there is a so-called proatlas, consisting of one or 

 two plates lying between the atlas and the skull. It is not known just 

 what relationship this bears to the other vertebrae. 



In fin-bearing animals, if the spinal column runs to the end of the 

 body in a straight line (Fig. 405), the caudal fins are known as diphycer- 

 cal, a condition found in the young of all fishes and in adult cyclostomes, 

 dipnoans, and crossopterygians. 



If, as in the elasmobranchs and ganoids, the tail axis bends abruptly 

 upward at the end, but retains the dorsal fin-part and a portion of the 



