MUSCULAR ACTIVITY 109 



the most favorable load is selected about half the energy of the 

 contraction may, as we learned above (p. 107), appear as mechani- 

 cal work. 



In the study of the muscle as an engine knowledge of the energy 

 relationships is of the greatest moment. We know that muscular 

 activity is based on chemical transformations, and to explain the 

 actions of the muscle we must know what these chemical trans- 

 formations are. If we assume any particular transformation to be 

 one from which the muscle derives its energy we must be able to 

 show that that transformation yields the amount of energy which 

 the muscle actually manifests. If it does not do so the assump- 

 tion which selected it as the source of the muscle's power is ob- 

 viously erroneous. 



There are two facts of the chemistry of muscular activity which 

 are fully demonstrated and which have already been stated. These 

 are the oxidation of sugar or fat as fuel, and the production of lactic 

 acid as an essential feature of the contraction process. Reference 

 has already been made to the view formerly held that the lactic 

 acid represents merely a stage in the oxidation of sugar, and to the 

 replacement of that view by the present one. The evidence upon 

 which is based this new recognition of the part played by lactic 

 acid rests in large part upon consideration of the energy relation- 

 ships. 



When a skeletal muscle is exposed to the vapor of chloroform 

 it passes into a condition of pronounced rigor. The strong con- 

 traction of rigor is the result of the production of lactic acid 

 within the muscle. It differs chemically from ordinary contrac- 

 tions of the living muscle in that the production of acid goes on 

 without concurrent oxidation of fuel, and in the fact that the lactic 

 acid produced is not immediately removed. Mechanically, as al- 

 ready pointed out, the rigor differs from ordinary contraction in its 

 persistence. The rigor contraction may last for hours, whereas 

 the ordinary contraction ends with the cessation of stimulation. If 

 . the muscle that is to be subjected to the action of the chloroform 

 is so firmly fixed at both ends that no actual shortening can occur 

 all the energy of the rigor contraction appears in the form of heat 

 and can be measured as such. Because there is no concurrent 

 oxidation all the energy thus manifested must be derived directly 

 from the chemical transformation which gives rise to the lactic 



