28 



PHYSIOLOGY AND TEMPERANCE. 



This is due to the shallowness of the joint, to its exposed 

 position, and to the varied functions of the arm. 



20. The Elbow. The elbow presents the best instance 

 of the forward and backward movements of a hinge. It is 

 formed by the lower end of the humerus and the upper ends 

 of the ulna and radius. The arm bone at its lower extremity 

 is wide and flat, while at its very end is a rounded, smooth 

 surface, which fits into a deep notch in the ulna. The radius 

 is quite small at this end, and forms very little of the elbow 

 joint. With its smooth head it rolls against the side of the 

 ulna as we turn the palm of the hand upward or downward. 



FIG. 8. The Wrist Joint. 



21. The Wrist. When we come to the wrist, we find the 

 relative size of the two bones is changed. Here the ulna is 

 quite small, and rolls in a similar way against the side of the 

 radius when the hand is turned. The end of the radius is 

 sufficiently large to form the whole upper surface of the wrist 

 joint. The bones of the wrist are small and mostly wedge- 

 shaped. They are arranged in two rows of four bones each, 

 and are so placed as to form a gentle curve when the wrist is 

 bent, giving it a graceful outline, instead of a sharp angle, as 

 in the ordinary hinge joint. 



22. The Hand. In the hand, including the thumb and 

 fingers, we see displayed the most perfect and complete mech- 



