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 



PIG. 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 hcemal 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 



