CHAP, ii.] THE CONTRACTILE TISSUES. 105 



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 (the pile being balanced so as to move with the contracting 

 muscle, and thus to keep the contact exact), or in the shape of a thin 

 wedge, 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. 



Another delicate method of determining the changes of temperature 

 of a tissue is based upon the measurement of alterations in electric 

 resistance which a fine wire, in contact with or plunged into the tissue, 

 undergoes as the temperature of the tissue changes. 



It has been calculated that the heat given out by the muscles of 

 the thigh of a frog in a single contraction amounts to 3'1 micro-units 

 of heat 1 for each gramme of muscle, the result being obtained by 

 dividing by five the total amount of heat given out in five succes- 

 sive single contractions. It will however be safer to regard these 

 figures as illustrative of the fact that the heat given out is consider- 

 able 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 ( 63) be regarded 

 as simple and direct oxidations, 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 combus- 

 tion 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 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 contraction 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 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 



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



