34 THE HUMAN BODY. 



siderable extent in any direction. On the other hand, 

 these pads so limit the movement that no sharp bend can 

 occur at any one point, such as might tear or bruise the 

 spinal cord lying in the neural canal. 



The sacral vertebrae grow together firmly to give a 

 solid support to the pelvic arch, which transmits the 

 weight of all the rest of the body to the lower limbs when 

 we stand. 



Summary. The back-bone is rigid enough to sup- 

 port all the rest of the body ; flexible enough to bend 

 considerably in any desired direction, yet not sharply at 

 any one point ; and elastic enough to destroy or greatly 

 diminish any sudden jar or jerk which it may receive. It 

 is one of the most beautiful pieces of mechanism in the 

 body. 



The ribs are twelve in number on each side (Fig. 15). 

 They are slender curved bones embracing the sides of the 

 chest, and attached at one end to the dorsal vertebrae. 

 Ventrally each rib ends in a costal cartilage; the carti- 

 lages of the seven upper pairs are directly articulated to 

 the sides of the breast-bone. The eighth, ninth, and 

 tenth cartilages join those of the ribs above them ; the 

 eleventh and twelfth are not attached to the rest of the 

 skeleton at their ventral ends, and are known as the free 

 or floating ribs. 



How is it that we can bend the backbone? How is the extent of 

 bending at any one point limited? Why? 



Why do the sacral vertebrae grow together? 



State briefly the mechanical properties of the vertebral column. 



How many ribs are there ? What is their shape ? To what are 

 their dorsal ends attached? How does each rib end ventrally? To 

 what are the costal cartilages of the first seven ribs attached? To 

 what the next three costal cartilages ? Which are the floating ribs? 

 Why so called? 



