140 
COMPARATIVE ZOOLOGY. 
the backbone is only partially ossified. But usually it 
consists of a number of separate bones, called vertebras, ar¬ 
ranged along the axis of the body. They range in number 
from 10 in the Frog to 305 in the Boa-constrictor. The 
skull, with its appendages, and the vertebrae, with the ribs 
and sternum, make up the axial skeleton. The shoulder 
and pelvic girdles and the skeleton of the limbs constitute 
the appendicular skeleton. 
A typical vertebra consists of a number of bony pieces 
so arranged as to form two arches, or hoops, connected by 
Fi«. 107.—Vertebrae—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; 6, laminae, or neurapophyses. 
a central bone, or centrum .' 16 The upper hoop is called 
the neural arch , because it encircles the spinal marrow; 
the lower hoop is called the haemal arch , because it en¬ 
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 neural 
canal , and above which is a process called the neural 
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 
