406 



THE PROPERTIES OF STRIPED MUSCIE. 



calories, i.e., the amount of heat corresponding to the excess of the negative over 

 the positive work, must be deducted, we have, as the actual amount of heat 

 produced by oxidation during the period in question — for a b, 118 microcalories ; 

 for b a, 63 microcalories. 



The question on which these experimental results throw light, is that 

 of the conditions which determine the expenditure of oxidisable material in 

 a muscle when in the excited state. With reference to this question, they 

 may be set forth as follows : — - 



From the data of experiments A and B, it is seen that during a 

 brief period of excitation (two seconds) the expenditure of the excited 

 muscle is greatest when the resistance opposed to its shortening is such as 

 it can overcome, so as to do its utmost amount of external work ; and that it 

 is least when the resistance is considerably greater than it can overcome, 

 i.e., when the opposing force of the load is sufficient to lengthen it to its 

 utmost. Between these two contrasting conditions comes that in which 

 the muscle neither prevails nor yields — that in which it is excited 

 isometrically. Here the heat produced must be less than in contraction 

 against resistance, greater than when the effort to contract is over- 

 powered by an antagonistic force — or, putting it otherwise, to hold on 

 against an opposing force involves expenditure, of which the amount is 

 proportional to the effort made ; but, provided that the effort remains 

 the same, to hold on against an unyielding resistance is less expensive 

 than to overcome an opposing force, more expensive than to be overcome 

 by it. 



From experiment C it appears that if an excited muscle is 

 allowed to shorten, and thereby to do work under the conditions 

 which we know to be most favourable, i.e., in lifting a diminishing 

 load, the expenditure of material which accompanies this effort is 

 affected in a remarkable way by antecedent mechanical conditions ; 

 if (case ah) it is excited when it is at its natural length, and at the 

 same moment its shortening is opposed by a decreasing load, of which 

 the initial amount is the greatest that it can lift, it expends much more 

 material than if (case b a), before beginning its effort, it is stretched from its 

 natural length when excited (x) to its natural length when relaxed (I). 

 The contrast between the two cases can only be explained by supposing 

 that the negative work, which in case o a has been done on the muscle 

 towards the end of the previous period of extension, leaves behind it 

 mechanical potential energy which, in the succeeding act of shortening, 

 takes the place of part of the mechanical energy which in case a b was 

 derived from the transformation of chemical energy. The theory of this 

 transformation can of course not be given, so long as the problem of the 

 origin of muscular force remains unsolved. 



The practical importance of both results lies in their bearing on the 

 conditions of exhaustion — meaning thereby that kind of exhaustion 

 which depends on the using up of oxidisable material. Experience 

 teaches that different kinds of muscular effort differ from each other in 

 a remarkable way in their relative aptness to induce fatigue, and that 

 this does not depend on the muscular tension with which the effort is 

 accompanied. Going up hill is exhausting because external work is 

 done against resistance. Descending is not exhausting, although the 

 muscles are as frequently in action, and as tense when in action as in 

 ascending. The reason why there is so little fatigue is that each 

 muscular effort is overcome by a superior opposing force — the weight of 



