70 THERMAL CHANGES. [BOOK i. 



Concerning the functional importance of these various bodies 

 we have very little exact knowledge. 



Helmholtz shewed long ago that the effect of long continued 

 contraction is to diminish the substances in muscle which are 

 soluble in water, but to increase those which are soluble in 

 alcohol. In other words, during contraction some substance or 

 substances soluble in water are converted into another or other 

 substances insoluble in water but soluble in alcohol. During or 

 after rigor mortis, glycogen is converted into sugar, and it has been 

 contended that a similar change takes place during contraction; 

 but we are not, at present at all events, in a position to affirm that 

 such a conversion is a necessary and integral part of the chemical 

 transformations which lie at the bottom of a muscular contraction. 



We shall have occasion to treat more fully and from a different 

 point of view, of the relations between muscular exercise and the 

 quantity of urea discharged by the kidneys. Meanwhile we may 

 state that not only does this all-important nitrogenous crystalline 

 body appear to be absent from normal muscle, both during rest 

 and after contraction, but we have as yet no adequate evidence 

 that the contraction of a muscle is followed by the appearance in 

 the substance of the muscle or in the blood passing through it of 

 any new nitrogenous product, or by any increase in any of the 

 nitrogenous extractives which we have mentioned as normally 

 present in muscle. In fact all we know at present is that a 

 contraction is followed by an increase in the discharge of carbonic 

 acid, and by certain changes which lead to an acid reaction. 

 Beyond this we are in the dark. 



Thermal Changes. 



The view however that chemical changes lie at the bottom of a 

 muscular contraction, that the energy which takes on the form of 

 muscular work arises from a metabolism of the muscular substance, 

 is supported by a variety of considerations and especially perhaps 

 by the fact, that the development of energy as muscular work, is 

 accompanied by a development of energy as heat. 



Though we shall have hereafter to treat this subject more fully, 

 the leading facts may be given here. Whenever a muscle contracts, 

 its temperature rises, indicating that heat is given out. When a 

 mercury thermometer is plunged into a mass of muscles, such as 

 those of the thigh of the dog, a rise of the mercury is observed 

 upon the muscles being thrown into a prolonged contraction. 

 More exact results however are obtained by means of a thermopile, 

 by the help of which the rise of temperature caused by a few 

 repeated single contractions, or indeed by a single contraction, may 

 be observed and the amount of heat given out approximative^ 

 measured. 



