234 PHYSIOLOGY 



cold irrevocably destroys its irritability. Warming the muscle will 

 now simply bring about rigor mortis. 



FATIGUE. A muscle will not go on contracting indefinitely. If 

 it be repeatedly stimulated, changes soon become apparent in the 

 curve of contraction. The latent period is prolonged, as well as the 

 length of the contractions ; the absolute height and work done are 

 diminished. At the same time the muscle does not return to its 

 original length the shortening which remains is spoken of as 



TIG. 69. Muscle curves showing fatigue in consequence of repeated stimu- 

 lation. The first six contractions are numbered, and show the initial 

 increase of the first three contractions. (BRODIE.) 



' contraction remainder* After an initial rise during the first few 

 contractions, these diminish uniformly in height till they are no 

 longer apparent, so that the muscle is now said to have lost its 

 irritability. 



At the same time there is a great prolongation of the curve, 

 occasioned almost entirely by a retardation of the relaxation, so that 

 after forty or fifty contractions several seconds may elapse before the 

 lever returns to the base line (Fig. 69). 



The fact that the relaxation part of the muscle curve is affected by various 

 conditions, especially fatigue, apparently independently of the contraction part, 

 led Fick to put forward a theory that two distinct processes were concerned in 

 the response of a muscle to excitation, one process causing the active shortening 

 and the other the relaxation. (It must be noted that this is not the same as 

 saying that the lengthening is an active process, a statement negatived by the 

 behaviour of a muscle when caused to contract on mercury.) He suggested that 

 the disintegration associated with activity might be conceived as occurring in two 

 stages : the first resulting in the production of sarcolactic acid and the active 

 shortening of the muscle ; the second in the further conversion of the acid into 

 C0 2 , with a consequent relaxation. A retardation of this second phase would 

 cause the prolonged curve with ' contraction remainder ' observed in a fatigued 

 muscle. The absence of any appreciable evolution of heat in the conversion of 

 glucose to lactic acid shows, however, that the formation of lactic acid cannot 

 account for the whole of the energy involved in the phase of shortening. 



If left to itself, the muscle which has been exhausted by repeated 



