EFFICIENT CA USES OF EXHA USTION. 389 



It must, however, be remembered that the diminution of the rate of 

 propagation in fatigue has not been proved experimentally. 



Efficient causes of exhaustion. — It was shown long ago by Ranke l 

 that the definition of exhaustion which we have characterised as in 

 the main correct — that which regards it as a state in which restitu- 

 tion lags behind waste — is not wholly adequate. Without denying 

 that mere disturbance of balance of nutrition in the muscle substance 

 may produce the effects of fatigue, he showed that in the frog 

 these may be got rid of by the circulation through the muscle 

 of normal salt solution, i.e. of a liquid which has no nutritive pro- 

 perties ; and that a state which cannot be distinguished from fatigue 

 can be produced in fresh muscles, by the injection of aqueous extract 

 of the fatigued muscles of another animal. 



Mosso 2 has shown much more recently, that even the introduction of 

 the blood of a fatigued animal (e.g. a clog) into the circulation of a 

 normal one will give rise in the latter to all the symptoms of fatigue 

 (respiratory, cardiac, etc.). It would appear, therefore, that some 

 toxic substance is produced in the muscles by tetanising them. 

 Ranke at first inferred from what was known at the time as to 

 the production of lactic acid in protracted muscular exertion, that 

 the acid was the true cause of exhaustion ; and he was thus led to 

 make the well-known experiments by which it was proved that 

 the introduction into the circulation of normal physiological salt 

 solution, containing a very small percentage of this acid, produces 

 exhaustion, and that the muscles regain their natural vigour when 

 the lactic acid is withdrawn, and the circulating fluid rendered 

 alkaline by a trace of sodium carbonate. He soon, however, began 

 to surmise that the fatiguing action belonged to lactic in common 

 with other acid bodies, and this led to the discovery that, even when no 

 free acid was present, a reaction due to the monophosphates of the 

 alkali metals was sufficient to produce the result. It was subsequently 

 found that the presence of lactates had no effect in a neutral solution, 

 so that exhaustion can only be attributed to lactic acid, in so far as the 

 production of this body is the immediate cause of the acid reaction of 

 fatigued muscle. The facts just stated prove no more than that acid 

 reaction is one of the conditions of exhaustion. It is not probable 

 that it is its only cause ; for it has been shown by Abelous as well 

 as by Mosso, that the alkaline serum of a fatigued animal also produces 

 fatigue. It cannot, therefore, be doubted that among the products of 

 muscular activity there exists a substance endowed with the property 

 of rendering muscle less competent, although we have as yet no sufficient 

 knowledge either of the nature or the mode of action of this substance. 



The remarkable fact insisted on by Waller, 3 that in the ordinary 

 process of exhaustion of muscle, the indirect excitability disappears 

 while the direct excitability persists, has been also shown to obtain 

 by Abelous 4 with fresh muscle (frog) when supplied with blood 

 derived from fatigued muscle. His method consisted in severing one 

 sciatic, and then throwing the whole muscular system with the exception 



1 "Grundzuge der Physiologie," S. 632. 



2 Trans. Intemat. Med. Cong.. Berlin, 1890. 



3 "Physiology," 2nd edition, p. 380. 



4 Abelous and Langlois, Arch, de physiol. norm, ctpath., Paris, 1893, tome v. pp. 437-448 ; 

 and 1894, tome vi. p. 432. 



