THE MECHANICAL RESPONSE OF MUSCLE 



till they are no longer apparent, so that the muscle is now said to have ] 

 its irritability. At the same time there is a great prolongation of the ra 

 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). 



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

 especially fatigue, apparently independently of the contraction part, led Pick to put 

 forward a theory that two distinct processes were concerned in the response of a 



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. (BEODIE.) 



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. We shall return to this point when discussing the chemical 

 and heat changes 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- 

 able for the supply of potential energy to this material. 



(2) The accumulation of waste products of contraction. Among these 

 waste products the lactic acid is probably of great importance. Fatigue 

 may be artificially induced in a muscle by ' feeding ' it with a dilute solutic 

 of lactic acid, and again removed by washing out the muscle with normal 

 saline solution containing a small percentage of alkali. 



