14 THE ANATOMY OF VERTEBRATED ANIMALS. 



Vertebrae which have centra concave at each end have 

 been conveniently termed amphiccelous ; those with a cavity 

 in front and a convexity behind, proccelous ; where the 

 position of the concavity and convexity is reversed, they are 

 opistliocoelous. 



In the Mammalia, the centra of the vertebrae are usually 

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

 developed from centres of ossification distinct from that of 

 the centrum itself. 



The centra of the vertebrae may be united together by 

 synovial joints, or by ligamentous fibres — the intervertebral 

 ligaments. The arches are connected by ligaments, and 

 generally, in addition, by overlapping articular processes 

 called zygapophyses, or oljlique 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 distract os odontoideum, or ankylosing with the 

 body of the axis, and becoming the so-caRed odontoid process 

 of this vertebra. 



In Vertebrata with well-developed hind limbs, one or 

 more vertebrae, situated at the posterior part of the trunk, 

 usually become peculiarly modified, and give I'ise to a 

 sacrwm, with which the pelvic arch is connected by the 

 intermediation of expanded and ankylosed ribs. In front 

 of the sacrum the vertebrae 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. 

 Vertebrae without distinct ribs, between the last dorsal and 

 the sacrum, are lumbar. Vertebrae, 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 inclose the backward continuation of the aorta, 

 and may be separately ossified as subcaudal, or' chevron, 

 bones. 



