THE SKIN AND SKELETON. 139 
spine. On the anterior and posterior edges of the arch 
are smooth surfaces, or zygapophyses, which in the natural 
state are covered with cartilage, and come in contact with 
the corresponding surfaces of the preceding and succeed- 
ing vertebrae. The bases of the arch are notched in front 
and behind, so that when two vertebrae are put together a 
round opening (intervertebral foramen) appears between 
the pair, giving passage to the nerves issuing from the spi- 
nal cord. From the sides of the arch, blunt transverse proc- 
esses project outward and backward, called diapophyses. 
Such are the main elements in a representative vertebra. 
The hzemal arch is not formed by any part of the verte- 
bra, but by the ribs and breast-bone. Theoretically, how- 
ever, the ribs are considered as elongated processes from 
the centrum (plewrapophyses), and in a few cases a he- 
mal spine is developed from the breast-bone correspond- 
ing to the neural spine. 
The vertebree are united together by ligaments, but 
chiefly by a very tough, dense, and elastic substance be- 
tween the centra. The neural arches form a continuous 
canal which contains and protects the spinal cord; hence 
the vertebral column is called the newroskeleton. The 
column is always more or less curved; but the beantiful 
sigmoid curvature is peculiar to Man. The vertebrae 
gradually increase in size from the head toward the end 
of the trunk, and then diminish to the end of the tail. 
The neural arch and centrum are seldom wanting; the 
first vertebra in the neck has no centrum, and the last in 
the tail is all centrum. The vertebra of the extremities 
(head and tail) depart most widely from the typical form. 
The vertebral column in Fishes and Snakes is divisible 
into three regions — head, trunk, and tail. But in the 
higher animals there are six kinds of vertebrae: cranial, 
cervical, dorsal, lumbar, sacral, and caudal. 
The cranial vertebrw form the skull. They are greatly 
