CHANGES IN A MUSCLE DURING CONTRACTION. 91 



will, however, be safer to regard these figures as illustrative of the fact that 

 the heat given out is considerable, rather than as data for elaborate calcula- 

 tions. Moreover, we have no satisfactory quantitative determinations of the 

 heat given out by the muscles of warm-blooded animals, though there can 

 be no doubt that it is much greater, than that given out by the muscles of 

 the frog. 



There can hardly be any doubt that the heat thus set free is the product 

 of chemical changes within the muscle changes which, though they cannot 

 for the reasons given above ( 63) be regarded as simple and direct oxida- 

 tions, yet, since they are processes dependent on the antecedent entrance of 

 oxygen into the muscle, may be spoken of in general terms as a combustion ; 

 so that the muscle may be likened to a steam-engine, in which the combustion 

 of a certain amount of material gives rise to the development of energy in 

 two forms, as heat and as movement, there being certain quantitative rela- 

 tions between the amount of energy set free as heat and that giving rise to 

 movement. We must, however, carefully guard ourselves against pressing 

 this analogy too closely. In the steam-engine we can distinguish clearly 

 between the fuel which, through its combustion, is the sole source of energy, 

 and the machinery, which is not consumed to provide energy, and only suffers 

 wear and tear. In the muscle we cannot with certainty at present make such 

 a distinction. It may be that the chemical changes at the bottom of a con- 

 traction do not involve the real living material of the fibre, but only some 

 substance manufactured by the living material and lodged in some way, we 

 do not know how, in the living material. It may be that when a fibre con- 

 tracts it is this substance within the fibre which explodes, and not the fibre 

 itself. If we further suppose that this substance is some complex compound 

 of carbon and hydrogen, into which no nitrogen enters, we shall have an 

 explanation of the difficulty referred to above ( 63), namely, that nitro- 

 genous waste is not increased by a contraction. The special contractile 

 carbon-hydrogen substance, may then be compared to the charge of a gun, 

 the products of its explosion being carbonic and sarcolactic acids, while the 

 real living material of the fibre may be compared to the gun itself, but to a 

 gun which itself is continually undergoing change far beyond mere wear and 

 tear, among the products of which change nitrogenous bodies like kreatin 

 are conspicuous. This view will certainly explain why kreatin is not 

 increased during the contraction, while the carbonic and lactic acids are. 

 But it must be remembered that such a view is not yet proved ; it may be 

 the living material of the fibre, as a whole, which is continually breaking 

 down in an explosive decomposition, and as continually building itself up 

 again out of the material supplied by the blood. 



In a steam-engine only a certain amount of the total potential energy of 

 the fuel issues as work, the rest being lost as heat, the proportion varying, 

 but the work rarely, if ever, exceeding one-tenth of the total energy, and 

 generally being less. In the case of the muscle we are not at present in a 

 position to draw up an exact equation between the latent energy on the one 

 hand, and the two forms of actual energy on the other. We have reason to 

 think that the proportion between heat and work varies considerably under 

 different circumstances, the work sometimes rising as high as one-fifth, some- 

 times possibly sinking as low as one-twenty-fourth of the total energy ; and 

 observations seem to show that the greater the resistance which the muscle 

 has to overcome, the larger the proportion of the total energy expended 

 which goes out as work done. The muscle, in fact, seems to be so far self- 

 regulating that the more work it has to do the greater, within certain 

 limits, is the economy with which it works. 



Lastly, it must be remembered that the giving out of heat by the muscle 



