Chap, ii.] THE CONTRACTILE TISSUES. 97 



how, in the living material ; it may be that when a fibre contracts 

 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 (§ 60), 

 namely, that nitrogenous 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 decom- 

 position, 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, or, according to some, as high even as one-half, some- 

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

 energy ; and observations seem to shew that the greater the re- 

 sistance 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 is not confined to the occasions when it is actually con- 

 tracting. When, at a later period, we treat of the heat of the body 

 generally, evidence will be brought forward that the muscles, even 

 when at rest, are giving rise to heat, so that the heat given out at 

 a contraction is not some wholly new phenomenon, but a temporary 

 exaggeration of what is continually going on at a more feeble 

 rate. 



Electrical Changes. 



§ 63. Besides chemical and thermal changes a remarkable 

 electric change takes place whenever a muscle contracts. 



