46 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



arch being completed by a neural spine as a keystone. Ventral 

 to the centrum is a similar haemal arch, composed, in like man- 

 ner, of haemapophyses and haemal spine, and enclosing, in the 

 caudal region, the caudal artery and vein, farther forward, the 

 coelom and viscera. This type of vertebra is common in many 

 fishes, and in the tails of some higher forms. In the lowest fishes 

 it is simplified by the omission of parts, while in the higher verte- 

 brates other structures are added. Among these are articular proc- 

 esses (zygapophyses) on the anterior and posterior faces of the neural 

 arch (distinguished by position as pre- and post-zygapophyses) 

 which lock the successive vertebrae together and strengthen the column 

 without interfering with its flexibility (fig. 38). 



FIG. 40. Diagrammatic sagittal sections of (.4) amphicoelous; (B), proccelus; (C), opistho 

 ccelous; and (D), amphplatyan vertebrae; the head supposed to be at the left. 



In all vertebrates above fishes most of the vertebrae bear transverse 

 processes (pleurapophyses) , extending laterally on either side. Of 

 these there are two kinds, a parapophysis arising from the centrum, 

 and a diapophysis projecting from the neural arch. The ribs articu- 

 late with the ends of these, as will be explained later. The distinctions 

 are the most marked in the lower vertebrates, but careful comparisons 

 show them in the mammals. Other processes, of less frequent occur- 

 rence, will be mentioned below in connection with the groups in which 

 they occur. 



The ends of the centra, where they articulate with each other, 

 may take five different shapes. They may be hollow at both ends 

 (amphicoelous); they may fit together with a ball and socket joint, 

 the hollow being sometimes in front (procoelous), sometimes behind 

 (opisthocoelous). In the mammals flat or amphyplatyan conditions 

 are common, while in birds saddle-shaped ends occur (figs. 40, 49). 



In the history of vertebrae both comparative anatomy and embryology agree 

 that the process of vertebral formation began with the arches and extended thence 



