THE MECHANICAL RESPONSE 'OF MUSCLE 



209 



ing which remains is spoken of as ' 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. 70). 



FIG. 70. Muscle curves showing fatigue in consequence of repeated stimulation. 

 The first six contractions are numbered, and show the initial increase of the 

 first three contractions. (BRODIE.) 



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



jcially 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 relaxa- 

 tion. (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 CO 2 , with a consequent relaxation. A retardation of this 

 second phase would cause the prolonged curve with ' contraction remainder ' observed 

 in a fatigued muscle. We shall return to this point when discussing the chemical 

 and heat chan'ges which accompany contraction. 



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

 tion will recover. The recovery is hastened by passing a stream of blood, 

 or even of salt solution, through the blood-vessels of the muscle. Recovery 

 in a muscle outside the body is never complete. 



The phenomena of fatigue probably depend on two factors : 

 (1) The consumption of the contractile material or the substances avail- 

 e for the supply of potential energy to this material. 

 (2) A more important factor is the accumulation of waste products of 

 contraction. Among these waste products the lactic acid is probably of great 

 mportance. Fatigue may be artificially induced in a muscle by ' feeding ' 

 t with a dilute solution of lactic acid, and again removed by washing out 

 ;he muscle with normal saline solution containing a small percentage of alkali. 





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