182 STUDIES IN PHYSIOLOGY 



The third class includes the gliding joints, and these are 

 usually formed of short bones. They glide over each other 

 in several directions and so allow a small range of move- 

 ment in many directions. 



KBGION OF JOINT JOINT OCCURS BETWEEN 



wrist radius and carpals. 



wrist the different carpals. 



knee kneecap and the femur, 



arch of foot the different tarsals and tlie 



metatarsals. 



head (both hinge and lower jaw and temporal. 



gliding) 



spinal column the different vertebrae, 



chest region vertebrae and ribs. 



In SL pivot joint one bone moves around a projection of the 

 other, the latter serving as a pivot. Such a joint is formed 

 between the first two vertebrae (see Fig. 67). The peg of 

 the axis, projecting upward through a hole in the atlas, is 

 the pivot about which the atlas 'turns. A rather more com- 

 plicated kind of pivot joint is formed between the radius and 

 a lower process of the humerus. The action of this joint 

 is easily demonstrated in the following manner: Extend 

 the forearm and the hand on the table with the palm up ; 

 then, without lifting the elbow from the table, turn the 

 hand over, so the palm is downward. In this movement 

 the lower (distal) end of the radius crosses the ulna, carry- 

 ing the hand along with it (see Fig. 78). It is because of 

 this rotating motion that the radius bone has received its 

 name. 



8. THE HYGIENE OF THE SKELETON 



Food and the Skeleton. In the composition of bones, we 

 found two kinds of matter, namely, animal and mineral. 

 For the growth of bones, therefore, it is essential that there 

 be a supply of both of these building materials. Bone cells, 



