220 MENTAL AND PHYSICAL EXERCISE. 

 Fig. 123. 



687. The easy motions of the hand, which might be 

 supposed to belong to the wrist, are in a great proportion 

 owing to the motions of the radius and ulna. The up- 

 and-down action of the hand, when the lower ends of 

 these bones are still, belong to the wrist, which is com- 

 posed of eight bones; but the rolling motions of the 

 hand, by which the palm is alternately turned up and 

 down, are caused entirely by the slight movements of 

 the radius on the ulna. The 'ulna projects beyond the 

 head of the humerus, forming, when the arm is bent, 

 the point of the elbow. The radius has a small round 



When the palm is turned up and down, what bones are concerned in the 

 motion ? 



