SKELETAL MUSCLE 77 



eliminating other elements from our problem. It is 

 important to point out that we cannot simplify the 

 conditions to the same extent when we deal with the 

 living body. The fatigue which we know by experience 

 cannot be assigned wholly to the muscles. We do not 

 use these in everyday life without using a large part of 

 the nervous system at the same time. We have, there- 

 fore, to admit that the flagging of our own powers as we 

 tire of any exercise may be owing to a decline of efficiency 

 on the part of the nervous elements. How many factors 

 we ought to take into account can hardly be estimated at 

 this time but will be more clear later. "A chain is no 

 stronger than its weakest link" and endurance in action 

 will be limited by the failure of any one of the several 

 mechanisms which are in cooperation. 



The Energy Transformations in Skeletal Muscle. 

 This is a difficult matter and we cannot pursue it far but 

 it would not be wise to omit all discussion of it. We 

 know that oxidation goes on and that motion results 

 but it has proved exceedingly hard to ascertain what 

 intermediate processes serve to connect the two. We 

 have hinted that the comparison between a muscle and 

 a steam engine is quite faulty and we must now show in 

 what chief respect. In the locomotive oxidation yields 

 heat and a part of the heat energy is presently trans- 

 formed to work. Heat arises in working muscles also 

 and is more or less proportional to the intensity of their 

 action. The temptation was to assume that in this case, 

 as in the other, all the energy set free existed at first as 

 heat and that a fraction of it, an instant later, was 

 applied to the performance of work. The earlier theories 

 were designed to show how heat might be converted into 

 movement by the muscle. 



Recent studies in which methods of extraordinary 

 refinement have been used seem to prove that most of 

 the heat developed in connection with the execution of 

 a simple contraction arises after the movement. The 

 inference is that the oxidation does not immediately 



