72 THE PRINXIPLES OF MYODYXAMICS. 



the fore-arm will have its force resolved into four 

 components in so far as the elbow-joint is con- 

 cerned, and into three components in so far as the 

 radio-ulnar-joint is concerned :— See Fig. 20. 



(I.) Let P be the contracting muscle, E its 

 origin, and m its insertion : Construct on m P E 

 the lono^ axis of the contracting; muscle, a rec- 

 tangular parallelo pipedon. The force of P will 

 evidently be resolved into the components a, b, 

 and c: — (i) The moving component will be a; 



(ii) the retentive component will be b in the 

 case of the radius, and a in case of the ulna ; 



(iii) the displacing component will be a in the 

 case of the radius, and b in the case of the 

 ulna ; (iv) and the lateral displacing component 

 will be c. 



(2.) The component a equals the component 

 e, and the component c equals the component 

 d : — But e is a component making in-rotation of 

 the radius, and may be called (i) a rotating coin- 

 ponent ; and this component tends to displace 

 the radius from the ulna, and may be called the 

 (ii) rotating displacing component ; while the 

 component d tends to hold the radius against 



