THE SKIN AND SKELETON 349 







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 a central bone, or centmm The upper 

 hoop is called the neural arch, because it encircles the 

 spinal cord; the lower hoop is called the hemal arck, 

 because it incloses the heart and the great central blood 

 vessels. An actual vertebra, however, is subject to so 



FIG. 304. 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; i, 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. 



many modifications, that it deviates more or less from 

 this ideal type. Selecting one from the middle of the 

 back for an example, 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 the correspond- 

 ing surfaces of the preceding and succeeding vertebrae. 

 The bases of the arch are notched in front and behind, 

 so that when two vertebrae are put together a round 



