CHAP, ii.] THE CONTRACTILE TISSUES. 71 



The thermopile may consist either of a single junction in the form of 

 a needle plunged into the substance of the muscle ; or of several junctions 

 either in the shape of a flat surface carefully opposed to the surface of 

 muscle (Heidenhain) the pile being balanced so as to move with the con- 

 tracting muscle, and thus to keep the contact exact ; or in the shape of a 

 thin wedge (Fick) the edge of which comprising the actual junctions is 

 thrust into a mass of muscles and held in position by them. In all cases 

 the fellow junction or junctions must be kept at a constant temperature. 



Fick calculated that the greatest heat given out by the muscles 

 of the thigh of a frog in a single contraction was 3*1 micro-units of 

 heat l for a gramme of muscle, the result being obtained by dividing 

 by five the total amount of heat given out in five successive single 

 contractions. It 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 calculations. 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 be regarded as 

 simple and direct oxidations, 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 relations between 

 the amount of energy set free as heat and that giving rise to move- 

 ment. 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 

 can make no such distinction ; though the whole matter is not 

 fully worked out, we have reason to think that the muscular fibre 

 is not to be regarded as a machine which takes so to speak a charge 

 of certain substances from the blood, and by inducing an explosion 

 of these substances in itself gives rise to the energy of heat and 

 movement. On the contrary the evidence goes to shew that it is 

 the living contractile substance 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 exceeding one-tenth of the 

 total energy. In the case of the muscle we are not at present in a 

 position to draw up an exact equation between the latent energy 



1 The micro- unit being a milligramme of water raised one degree centigrade. 



