THE FOREARM. 



305 



bones lie parallel to one another and widely separated (Fig. 321). In the midposi- 

 tion the radius lies above the ulna and the space between them is at its maximum. 

 The difference in this respect between midpronation and complete supination is 

 slight. The head of the radius rotates in the orbicular ligament, the lower end of 

 the radius revolves around the head of the ulna and rests on the interarticular trian- 

 gular fibrocartilage. The range of movement is from 140 degrees to 160 degrees. 

 The radius is pronated by the pronator teres and pronator quadratus muscles. It 

 is supinated by the brachioradialis, supinator (brevis), and biceps muscles. Some 

 of the other muscles also aid slightly in these movements, especially the flexor carpi 

 radialis in pronation. In fractures the preservation of the interosseous space is 

 essential for the proper performance of pronation and supination; hence anything 



FIG. 319. The axis of rotation 

 in pronation and supination. 



FIG. 320. Position of the bones 

 of the forearm when the hand is in 

 the position of pronation. 



FIG. 321. Position of the 

 bones of the forearm when the hand 

 is in the position of supination. 



which tends to encroach on it, as displacement of the fragments or their position as 

 influenced by the position of the hand, is to be guarded against. 



The muscles of supination are much stronger than those of pronation ; for this rea- 

 son instruments intended to be used in a rotary manner turn from the inside toward 

 the outside; that is, in the direction of supination. The screw-driver is an example. 



MUSCLES OF THE FOREARM. 



The movements of the hand and fingers are so intricate and complex as to 

 necessitate a large number of muscles for their performance. It is probably easiest 

 in order to understand the construction of the forearm to study these muscles in 

 reference to their functions. 



The muscles which occupy the forearm form three groups, which have separate 

 functions: (i) to flex and extend the fingers; (2) to flex and extend the wrist; (3) 

 to pronate and supinate the hand. 



