vn THE CHEMISTRY OF MUSCLE 297 



evolution of heat or of carbonic acid, but the muscle 

 would become acid in reaction due to the production of 

 lactic acid. The obvious chemical changes which usually 

 accompany-the contraction of muscle are those involved 

 in the manufacture of fresh explosive material. This 

 material is a store of energy and is rebuilt at the expense of 

 the active oxidation of other substances, of which probably 

 sugar is usually the most important. The evidence of 

 this oxidation consists of (1) the using up of sugar from 

 the blood, (2) the using up of oxygen from the blood, 

 (3) the discharge of carbonic acid into the blood, (4) the 

 evcjlution of heat. When the supply of oxygen and 

 nourishment to the muscle is sufficient, and the exercise 

 is not excessive as in the case of the heart beating with 

 its normal rhythm, there is no reason to suppose that any 

 considerable quantity of acid such as lactic acid leaves the 

 muscle. 



In the absence of oxygen, or in the presence of a 

 deficient supply of oxygen the reaction of the muscle 

 becomes more acid, and this acidity has an effect on what 

 we have spoken of as the protein fabric of muscle. At 

 first this becomes less mobile, so that the muscle contracts 

 with greater difficulty and under tliese conditions we say 

 that the muscle is fatigued — a condition which must be 

 distinguished from nervous fatigue or partial loss of 

 " grip " over our nmscles. Ultimately, if the muscle were 

 persistently deprived of oxygen, the acid reaction would 

 become so pronounced as to cause actual precipitation of 

 the protein fabric ; the muscle would then become 

 opaque and lose its contractility much as it does when 

 boiled. On stopping the circulation and thus cutting ofi" 

 the supply of oxygen these changes take place rapidly if 

 the muscle is stimulated, slowly if it is not. In either 

 case they spell death to the muscle and are a constant 

 phase of the death of the individual. This coagulation of 

 the protein fabric of the muscle on death is known as 

 "rigor mortis." 



