40 GENERAL ANATOMICAL CHARACTERS 
of the trunk, in nearly all mammals which possess completely 
developed hinder limbs, two or more vertebree become ankylosed 
together to form the “sacrum,” or portion of the vertebral column 
to which the pelvic girdle is attached; (2) in some species of 
Whales and Armadillos there are constant ossific unions of certain 
vertebrae of the cervical region. 
Although the vertebre of different regions of the column of the 
same animal or of different animals present great diversities of 
form, yet there is a certain general resemblance among them, or a 
common plan on which they are constructed, which is more or less 
modified by alteration of form or proportions, or by the addition or 
suppression of parts to fit them to fulfil their special purpose in the 
economy. An ordinary or typical vertebra consists, in the first 
place, of a solid piece of bone, termed the body or centrum (Fig. 
9, ¢), of the form of a disk or short cylinder. The bodies of con- 
Fia. 9.—Anterior surface of Human 
thoracic vertebra (fourth). c, Body or 
centrum; nc, neural canal; , pedicle, 
and J, lamina of the arch; ¢, transverse 
process ; az, anterior zygapophysis. 
Fic. 10.—Side view of the first lum- 
bar vertebra of a Dog (Canis familiaris). 
s, Spinous process ; az, anterior zygapo 
physis ; pz, posterior zygapophysis ; m, 
metapophysis ; a, anapophysis; ¢, trans- 
verse process. 
tiguous vertebre are connected together by a very dense, tough, and 
elastic material called the “intervertebral substance,” of peculiar and 
complex arrangement. This substance forms the main, and in some 
cases the only, union between the vertebrae. Its elasticity provides 
for the vertebree always returning to their normal relation to each 
other and to the column generally, when they have been disturbed 
therefrom by muscular action. A process (») arises on each side 
from the dorsal surface of the body. These processes, meeting in 
the middle line above, form an arch, surmounting a space or short 
canal (nc). Since it contains the posterior prolongation of the 
great cerebro-spinal nervous axis, or spinal cord, this space is called 
the neural canal, and the arch the neural arch, in contradistinction 
to another arch on the ventral surface of the body of the verte- 
bree, called the hemal arch. The latter is, however, never formed 
