68 HUMAN PHYSIOLOGY 



chemical changes in muscle therefore govern both the intake 

 and output of matter from the body. 



By studying the question from a number of different 

 standpoints, and by comparing the results so obtained, a 

 fairly clear conception of the chemical changes and the 

 source of muscular energy has been obtained. 



1. Composition of Muscle before and after Contraction. 

 The method which most naturally presents itself is to take 

 two muscles or groups of muscles corresponding to one 

 another, and to examine the chemistry of one before it had 

 been made to contract, and of the other after it had been 

 contracting for some time. 



Resting muscle is alkaline ; but if an excised muscle, out- 

 side the body, be kept contracting for some time, it becomes 

 acid, and this acidity is due to the appearance of sarcolactic 

 acid. Muscle in the body does not become acid, because 

 the alkaline lymph at once neutralises the acid which is 

 produced. 



Again, after contraction, the glycogen of the muscle is 

 found to be diminished. But the most important change is 

 that the amount of carbon dioxide, CO 2 , which can be ex- 

 tracted from, muscle is very greatly increased. 



As yet the changes, if any, in the proteids of muscle 

 during contraction have not been fully investigated, while 

 the results of the work accomplished on the nitrogenous 

 extractives, which are formed by the decomposition of the 

 proteids, are not trustworthy. They seem to indicate that 

 these bodies are increased during muscular contraction in 

 the excised muscle. These changes in a piece of muscle 

 may be diagrammatically represented as follows : 



-f Carbon dioxide. 



+ Sarcolactic acid. 



+ Nitrogenous extractives ? 



Glycogen. 



The results obtained by this method of investigation are 

 thus of considerable value, but alone they give us no clear 

 idea of the nature of the chemical changes. 



