46 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
arch being completed by a neural spine as a keystone. Ventral 
to the centrum is a similar hemal arch, composed, in like man- 
ner, of hemapophyses and hemal 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 vertebre together and strengthen the column 
without interfering with its flexibility (fig. 38). 
Fic. 40.—Diagrammatic sagittal sections of (A) amphiccelous; (B), proccelus; (C), opistho 
ceelous; and (D), amphplatyan vertebre; the head supposed to be at the left. 
In all vertebrates above fishes most of the vertebre 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 
(amphiccelous); they may fit together with a ball and socket joint, 
the hollow being sometimes in front (proccelous), sometimes behind 
(opisthocelous). In the mammals flat or amphyplatyan conditions 
are common, while in birds saddle-shaped ends occur (figs. 40, 49). 
In the history of vertebra both comparative anatomy and embryology agree 
that the process of vertebral formation began with the arches and extended thence 
