THE SKIN AND SKELETON. 
141 
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 
spinal cord. From the sides of the arch, blunt transverse 
processes project outward and backward, called diapophy¬ 
ses. Such are the main elements in a representative ver¬ 
tebra. The haemal arch is not formed by any part of the 
vertebra, but by the ribs and breast-bone. Theoretically, 
however, the ribs are considered as elongated processes 
from the centrum {pleurapophyses), and in a few cases a 
haemal spine is developed corresponding to the neural 
spine. 
The vertebrae 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 neuroskeleton. The 
column is always more or less curved; but the beautiful 
sigmoid curvature is peculiar to Man. The vertebrae 
gradually increase in size from the head towards 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 vertebrae 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 vertebra* form the skull. 77 They are greatly 
modified, as the neural arches are expanded to enclose the 
brain. The number of distinct bones composing the skull 
is greatest in Fishes, and least in Birds: this arises partly 
