140 
COMPARATIVE ZOOLOGY. 
the backbone is only partially ossified. But usually it 
consists of a number of separate bones, called vertebrae, 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 
B. 
Fig. 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 ; b , laminae, 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 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 bacic 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 smootli surfaces, or zygapophyses , which in the natural 
state are covered with cartilage, and come in contact with 
