8 COMPARATIVE PHYSIOLOGY 



phenomenon of fatigue, in which the muscle is unable to 

 recover its original reactivity between successive stimulation ; 

 and that the oxidative removal of lactic acid is an essential 

 feature of the recovery process by which the state previous to 

 excitation is restored. More recently Meyerhoff has shown 

 that the appearance of lactic acid in muscle is correlated v/ith 

 the disappearance of a corresponding amount of glycogen, a 

 hexose diphosphate being an intermediate compound in the 

 transformation. The appearance of the lactate ion in the 

 contraction process must not be taken to imply that there is 

 any observable increase in hydrogen ion concentration in a 

 single twitch or short tetanus ; Ritchie finds that there is not. 

 The presence of free lactic acid must ordinarily be instan- 

 taneous, as one would expect in a buffered system such as that 

 which exists in tissues. Since the mechanical relaxation of 

 muscle occurs as well in the absence of oxygen, this part of the 

 recovery may be assumed to correspond with the immediate 

 neutralisation of the lactic acid set free at excitation. Further 

 light can be obtained on this question by considering heat 

 production in muscular contraction. 



(c) Heat Production in Muscle.— That the temperature of 

 a muscle rises during contraction is easily demonstrated by 

 stimulating living and dead muscle in contact with the metallic 

 junctions of a thermopyle placed in circuit with a sensitive 

 galvanometer. The extension of our knowledge of the heat 

 production of muscle in recent years is chiefly due to the work 

 of A. V. Hill. Two questions are of pre-eminent interest as 

 throwing light on the mechanism of muscular contraction, 

 namely, the relation of the heat produced to the chemical events 

 of the contraction cycle, and the relation of heat-production 

 to the potential energy for mechanical work set free at 

 excitation. 



Hill has formulated the first of these two issues in the 

 following way : Is the heat-production of muscle given out 

 in some process by which the tension of muscle is increased, 

 or in the recovery process by which the mechanism is restored 

 to its original condition ? A satisfactory answer to this 

 question is obtained by studying the time-relations of heat 



