THE PRINCIPLES OF MYODYNAMICS. J 5 



let the ulna be fixed in this position : As the 

 radius rotates completely in-and-out, the inser- 

 tion of the supinator longus will describe a 

 semi-circle, and it is evident that the top of this 

 semi-circle will be nearer the origin of the 

 supinator longus than any other points. Hence, 

 under the circumstances, the supinator longus 

 will shorten from complete supination to mid- 

 rotation, and elongate from mid-rotation to com- 

 plete pronation ; but all the time the supinator 

 longus will act as a flexor of the forearm. 



(3.) In the third place, the moving component 

 of the supinator longus tends to displace the 

 radius from the ulna — especially the base of the 

 radius from the head of the ulna. 



63. The supinator longus is properly a flexor 

 of the radius — even as much as the biceps 

 brachii : 



(i.) In a given case, the length of the radius 

 was nine inches, and the length of the power- 

 arm of the bony lever was one and one-fourth 

 inches — the power being the biceps brachii. 

 The force of the biceps brachii must be 36a 

 pounds to lift 5o pounds. 



