138 COMPARATIVE ZOOLOGY. 
rupeds. It should be remembered, however, that all Ver- 
tebrates are formed on one plan. 
The vertebrate skeleton consists of a series of rings, 
called vertebrw, arranged along the back of the body, 
with certain appendages, as limbs, ribs, ete. The verte- 
bre are always present, but the appendages are incon- 
stant. In the lowest Fishes, the spinal column is a con- 
tinuous cylinder, with scarcely a trace of division, and in 
Birds, also, it is much consolidated ; but usually the verte- 
bree are separable. They range in number from 10 in the 
Frog (not counting the head) to 305 in the Boa-constrictor. 
A typical vertebra consists of a number of bony pieces 
so arranged as to form two arches, or hoops, connected by 
Fig. 107.—Vertebrree—A, cervical; B, dorsal: 2, centrum; 4, transverse process, con- 
taining foramen, a, for artery; 5, articular process; 3, spinous process, or neural 
spine; 1, neural canal; 6, facets for head of rib, the tubercle of the rib fitting 
in a facet on the process, 4; b, laminz or neurapophyses. 
a central bone, or centrum.” The upper hoop is called 
the neural arch, because it encircles the spinal marrow ; 
the lower hoop is called the hemal arch, because it in- 
closes the heart and the great central blood-vessels. An 
actual vertebra, however, is subject to so many modifica- 
tions, that it deviates more or less from this ideal type. 
Selecting one from the middle of the back for an exam- 
ple, we see that the centrum sends off from its dorsal side 
two branches, or processes, called neurapophyses. These 
meet to form the neural arch, under which is the newral 
canal, and above which is a process called the newral 
