SHORTENING OF INDIVIDUAL MUSCLES. 



245 



in such a position that the weight is pulled up towards the staple as 

 far as it will go. The length of the thread from hook to staple gives 

 the length of the muscle at its fullest physiological extension. The 

 weight is then allowed to pull the hook towards the staple, and the 

 distance in this case gives the length of the muscle in complete physio- 

 logical contraction. The difference between these two lengths is equal 

 to the shortening which takes place during contraction. 



The terms full physiological extension and full physiological con- 

 traction have been used to indicate that the measurements relate to the 

 extensions and contractions which can actually take place in the living 

 body. An excised muscle can of course be extended beyond the point 

 to which it is stretched, when it is physiologically extended within the 

 body. In the same way, an excised muscle can contract to a shorter 

 length than it ever reaches during life. A muscle in situ is in fact 

 always stretched to a certain degree ; thus when we flex the shoulder, 

 rotate it inwards, bend the elbow, and supinate the hand, the points of 

 attachments and origin of the biceps muscle are as close together as 

 they will come. The muscle can, however, in this position be made to 

 contract by an effort of the will, and this contraction can be felt by the 

 other hand. 



Weber 1 was the first to investigate the amount of shortening which takes 

 place during the contraction of the individual muscles of the body. 



The following table gives some of the measurements of E. Fick 2 : — 



Work done by a contracting muscle. — When a force acts upon a 

 body and moves it, that force is said to perform work, which work is 

 measured by the product of the force into the space traversed in the 

 direction of the force — 



lF = .Fx,S'. 



When a muscle contracts, its force is capable of lifting a body 

 against gravity, and, as has already been shown, the muscular force is 

 measured as against that of gravity itself. If a muscle contracts and 

 lifts a unit of mass, 1 grm., the force employed in dynes is 



1 x 981 (the acceleration due to gravity). 



If the gramme be lifted through unit of space (1 cm.), the work 



1 Ber. d. k. Sachs. Gesellsch. d. Wissensch., Math.-Phys. CI., 1851. 



2 E. Fick, VcrhandL d. phys.-med. GcscUscli. in Wurzburij, N. F., Bd. xi. S. 340. 



