166 



STUDIES IN PHYSIOLOGY 



meet corresponding processes on other vertebrse, they allow 

 these bones to move or articulate upon each other. 



The Structure of Atlas and Axis. The two top vertebrse 

 have a peculiarly modified structure, which allows the nod- 

 ding and turning movements of the head. The skull rests 

 upon the first cervical vertebra ; it is called the atlas (Fig. 



67, a). This name was sug- 

 gested by one of the myths 

 of ancient history, in which 

 the hero Atlas is said to 

 have supported the world 

 on his shoulders. When we 

 nod, in saying "yes," the 

 skull rocks backward and 

 forward on the atlas. In 

 signifying "no," on the 

 other hand, the atlas turns 

 about a projecting peg (Fig. 

 67, c) on the top of the 

 second vertebra (Fig. 67, &). Hence, the second cervical 

 vertebra is called the axis. When the head is tilted from 

 side to side, the motion involves several of the cervical 

 vertebrse. 



Adaptations shown in the Spinal Column. The human 

 spinal column is a wonderful piece of mechanism, which by 

 its structure is adapted to perform at the same time three 

 distinct functions. In the first place, we have seen that the 

 bodies of the vertebrce, piled one on the other, form a column 

 strong enough to support the weight of the body. In the neck 

 region the vertebrse are relatively small (Fig. 65) ; but 

 through the dorsal region their size increases, until in the 

 last lumbar vertebra we find the largest centrum in the series. 

 The spinal column, therefore, forms a cone with the base 

 made by the five sacral bones united into one ; to this solid 

 base the legs are attached. 



Again, the structure of the spinal column shows marvel- 



FIG. 67. Atlas and Axis Vertebrse. 



= atlas, on which rests the skull. 

 b = axis, about which atlas turns, 

 c = peg of axis projecting upward 

 through hole in atlas. 



