A STUDY OF THE SKELETON 



167 



ous provisions for securing elasticity and freedom of motion. 

 Elasticity is secured by a succession of four curves which 

 are best seen in a side view of the body (Fig. 65, A). By 

 means of these curves the head and the upper part of the 

 trunk are saved from 

 sudden shocks that would 

 result from running or 

 jumping, for the curves 

 act like a series of 

 springs. The cartilage 

 pads, likewise, serve as 

 cushions to prevent jar- 

 ring. That a consider- 

 able range of movement 

 is allowed by the struc- 

 ture of the vertebral col- 

 umn will be evident after 

 a trial by each student. 

 With the heels together, 

 and the hips held firm, 

 one can bend the body 

 forward and backward, 

 and from side to side; 

 and the whole pile of 

 bones can also be twisted 

 around sufficiently to al- 

 low one to look behind 



FIG. 68. Spinal Column, Ribs, and Ster- 

 num. 



a-b = spinal column. 



c= breastbone (sternum). 

 d = cartilage of true rib. 

 e = united cartilages of false ribs. 

 1-7 = true ribs with distinct cartilages. 

 8-10 = false ribs, cartilages united. 

 11-12 = floating ribs. 



oneself. 



A third adaptation that 

 is evident in the structure 

 of the spinal column is the protection it affords for the delicate 

 spinal cord. The series of neural arches and vertebral bodies 

 form a more or less continuous tube, within which lies the 

 spinal cord. Connected with this part of our nervous system 

 are thirty-one pairs of spinal nerves, which pass out from 

 the sides of the tube in which the cord lies by holes between 



