18 THE ANATOMY OF VERTEBEATED ANIMALS. 



In the Mammalia, the centra of the vertebras are usually 

 flat at each end, the terminal faces being discoidal epiphyses, 

 developed from centres of ossification distinct from that of the 

 centrum itself, 



The centra of the vertebras may be united together by 

 synovial joints, or by ligamentous fibres — the intervertebral 

 ligaments. The arches are connected by ligaments, and gen- 

 erally, in addition, by overlapping articular processes called 

 zygapophyses, or oblique processes. 



In a great many Vertebrata, the first and second cervical, 

 or atlas and axis, vertebrae undergo a singular change ; the 

 central ossification of the body of the atlas not coalescing 

 with its lateral and inferior ossifications, but either persist- 

 ing as a distinct os odontoideu7n,OT an chy losing with the body 

 of the axis, and becoming the so-called odontoid process of 

 this vertebra. 



In Vertebrata with well-developed hind-limbs, one or more 

 vertebras, situated at the posterior part of the trunk, usually 

 become peculiarly modified, and give rise to a sacrum, with 

 which the pelvic arch is connected by the intermediation of 

 expanded and anchylosed ribs. In front of the sacrum the ver- 

 tebrge are artificially classed as cervical, dorsal, and lumbar. 

 The first vertebra, the ribs of which are connected with the 

 sternum, is dorsal, and all those which lie behind it, and have 

 distinct ribs, are dorsal. Vertebrje without distinct ribs, 

 between the last dorsal and the sacrum, are lumbar. Ver- 

 tebras, with or without ribs, in front of the first dorsal are 

 cervical. 



The vertebrae which lie behind the sacrum are caudal or 

 coccygeal. Very frequently, downward processes of these 

 vertebrae enclose the backward continuation of the aorta, and 

 may be separately ossified as subcaudal, or chevron, bones. 



A tolerably complete segment of the spinal skeleton may 

 be studied in the anterior part of the thorax of a crocodile 

 (Fig. 5). It presents a procoelous vertebral ce7itrum (C), 

 united with which by the neurocentral suture is the neural 

 arch, which rises into the neural spine (iV S.). Two pro- 

 cesses, the prezygapophyses {Z), extend from the front part of 

 the arch, and have flat articular surfaces turned dorsally. Two 

 others of similar form, but having their articular surfaces 

 turned ventrally, proceed from the posterior face of the 

 neural arch, and are the postzyg apophyses {Z"). By these, 

 which are often called oblique, or articular, processes, the ^er- 

 tcbra articulates with the corresponding processes ofits prede 



