190 



COMPARATIVE PHYSIOLOGY. 



return to its original length ; and if a series of weights which 

 differ by a common increment be applied in succession and the 

 degrees of extensions compared, as may be 

 done by the graphic method, it will be ap- 

 parent that the increase in the extension 

 does not exactly correspond with incre- 

 ment in the weight, but is proportionally 

 less. With an inorganic body, as a watch- 

 spring, this is not the case. 



Further, the recoil of the muscle ailev 

 the removal of the weight is not perfect 

 for all weights ; but within certain narrow 

 limits this is the case, i. e., the elasticity 

 of muscle, though slight (for it is easily 

 over-extended), is perfect. When once a 

 muscle is over-extended, so weighted that 

 it can not reach its original length almost 

 at once, it is very slow to recover, which 

 explains the well-known duration of the 

 effects of sprains, no doubt owing to some 

 profound molecular change associated with 

 the stretching. 



The tracings below show at a glance 

 the difference between the elasticity of 

 muscle and of ordinary bodies. 



It is a curious fact that a muscle during 

 the act of contraction is more extensible 

 than when passive ; a disadvantage from 

 a purely physical point of view, but prob- 

 ably a real advantage as tending to obviate 

 sprain by preventing too sudden an appli- 

 cation of the extending force. 



It will be borne in mind that the limbs 



S™ rod"' attach!? to are held together as by elastic bands slight- 

 wiTaiens""^"'"^'™'* 1^ «" t^^e stretqh, owing to the elasticity 

 of the muscles. Now, as seen in many 

 tracings of muscxilar contraction, there is a tendency to imper- 

 fect relaxation after contraction — the contraction remainder 

 or elastic after-effect, which can be overcome by gentle trac- 

 tion. In the living body, the weight of the limbs and the action 

 of the stretched muscles on the side of the limb opposite to that 

 on which the muscles in actual contraction are situated, com- 



FiG. 178. — Dn Bois-Key- 

 mond'B apparatus for 

 the stud/ of elastic ex- 

 tension in muscle (after 



