346 HEAT DEVELOPED BY CONTRACTING MUSCLE. [BOOK I. 



When muscle becomes exhausted, both the work done and the heat 

 generated decline; but the latter more quickly than the former 1 . The 

 cause of this dissimilarity may be one or other of the two following. It 

 may be that the heat-evolving and the work-evolving appliances in 

 muscle are totally distinct, and variously affected by the same condi- 

 tions. Or it may be that the heat and mechanical work of muscle, like 

 the heat and mechanical work of a steam-engine, arise in a common 

 fundamental combustion ; and that the relative proportions of the 

 two are to some extent determined by external conditions, just 

 as some steam-engines work more economically than others, i.e. with 

 a larger proportionate yield of mechanical work 2 . 



It is impossible to say whether heat-developing processes are oc- 

 curring during the latent period 3 ; but there is little doubt that they 

 continue beyond the period of maximum contraction. It is at least 

 certain that the heat developed in a muscle is influenced by the load 

 which it bears during relaxation as it is by that which it bears in 

 contraction ; and such influence cannot be explained as the result 

 of mere forcible extension 4 . 



The proportion of heat and work evolved in contraction has been 

 determined by Fick 5 in the case of excised frog-muscles to vary 

 according to the load ; the greater the load the larger the proportion 

 of the total actual energy taken up by mechanical work. Under the 

 most favourable circumstances for the performance of mechanical 

 work the relation of work to heat was 1 : 3'8, and in the least 

 favourable of Fick's experiments the relation was 1 : 23'(?. It is 

 extremely uncertain how far these fractions can be applied to muscles 

 within the body, or to the muscles of warm-blooded animals. 



Fick's demonstration of this interesting relationship depends upon the 

 fact that, when the motion of a falling body is suddenly arrested, an 

 amount of heat appears, equivalent to the mechanical motion destroyed. 

 By direct experiment he proved that, if a weight suspended from a muscle 

 is raised by external means to a certain height and then let fall, the muscle 

 suffers a heating proportionate to the fall, i.e. which is the precise 

 equivalent of the work done in lifting the weight. He therefore caused 

 a loaded muscle to contract and afterwards allowed it to re-extend under 

 the weight which it had lifted; and then observed by how much the 

 temperature of the muscle had been raised. From the specific heat of 

 muscular tissue he was able to calculate the total quantity of heat gained 

 by the muscle in the process; and by subtracting from this total the heat- 

 equivalent of the work done in raising the weight, he was able to compare 

 the heating of a muscle under a certain load and the work done in raising 

 the load. 



1 Heidenhain, Mechanische Leistung, etc. p. 74. 



2 See Hermann's Handbuch der Physiol, Ed. i. Abth. i. p. 168. 



3 Nawalichin, Loe. cit. Pfliiger's Archiv, Vol. xiv. p. 311. 



4 Steiner, Loc. cit. Pfliiger's Archiv, Vol. xi. p. 204. See also Heidenhain with 

 Landau and Pacully, Loc. cit. Pfliiger's Archiv, n. p. 423. 



5 Fick, Loc. cit. Pfliiger's Archiv, Vol. xvi. p. 79. 



