THE TISSUES 65 



two muscles or groups of muscles corresponding to one 

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

 been made to contract, and of the other after it has 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 carbon 

 dioxide and 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 ylycogen of the muscle is 

 found to be diminished. But the most important change is 

 that the amount of carbon dioxide, C0 2 , which can be ex- 

 tracted from muscle is very greatly increased. 



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

 during contraction have not been fully investigated, and 

 the results of the work accomplished on the nitrogenous 

 extractives, which are formed by the decomposition of the 

 proteins, are not trustworthy. They seem to indicate that 

 these bodies are increased during muscular contraction in 

 the excised muscle. These changes in a muscle may be diagram- 

 matically represented as follows : 



+ 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. 



2. Respiration of Excised Muscle. By enclosing the excised 

 muscle in a closed space containing air of known composition, 

 and by investigating the changes in the components of the air 

 after the muscle has either been kept at rest for some time or 

 made to contract, important light has also been thrown on 

 these chemical changes. 



It has been found that the resting muscle constantly takes 



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