214 THE PHYSIOLOGY OF MUSCULAR WORK 



been greatly extended. Their results are given in the following 

 summary taken from their paper : 



" Freshly excised resting muscle is found to yield very small 

 quantities of lactic acid, and these small amounts are possibly not 

 more than can be accounted for by the unavoidable minimum of 

 manipulation prior to extraction. 



" A large increase of the yield of lactic acid is found as the result 

 of mechanical injury, of heating, and of chemical irritation. 



" Lactic acid is spontaneously developed under anaerobic con- 

 ditions in excised muscles. During the survival periods of sub- 

 sisting irritability, and not after, equal increments of acid arise 

 in equal times. After complete loss of irritability the lactic acid 

 yield remains stationary. 



" Fatigue due to contractions of excised muscle is accompanied 

 by an increase of lactic acid. The amount of acid attainable by 

 severe direct stimulation is found, with notable constancy, to be 

 not more than about one-half of that reached in the production 

 of full heat-rigor, or by the action of other destructive agencies 

 than heat. 



" In an atmosphere of oxygen there is no survival development 

 of lactic acid for long periods after excision. 



" From a fatigued muscle, placed in oxygen, there is a disappear- 

 ance of lactic acid already formed. 



" This disappearance of lactic acid due to oxygen does not occur, 

 or is masked, at supra-physiological temperatures. It is not found 

 in muscle which has suffered mechanical injury ; one essential 

 condition for this effect of oxygen appears to be the maintenance 

 of the normal architecture of the muscle. 



" The amount of lactic acid produced in full heat-rigor is con- 

 stant for similar muscles. This ' acid maximum ' of heat-rigor is 

 not affected by a previous appearance within the excised muscle 

 of lactic acid due to fatigue, or by a previous disappearance of 

 acid in the presence of oxygen, or by alternate appearances and 

 disappearances several times repeated." 



It will be necessary later to consider this work in its bearing 

 upon the effects of muscular activity, upon hyperpncea and muscle 

 soreness ; at the present time only the chemical characteristics 

 of muscle have to be discussed. 



The nitrogenous extractives of muscle form a large group ; the 

 chief ones are creatin, carnic acid, inosinic acid, carnosin and purin 



