30 THE BEGINNINGS OF LIFE. 



c All these experiments agree in showing that in the 

 muscular system of an animal which accomplishes ac- 

 tual work (such as raising a weight, dragging a load, &c.) 

 everything goes on as in an ordinary steam-engine. 

 Whilst the muscle performs <work^ the heat produced by 

 the internal combustions becomes divided into two 

 complementary portions; the one part appears as sen- 

 sible heat, and determines the temperature of the muscle, 

 the other disappears^ so far as its existence in the form 

 of heat is concerned, and, by the intervention of the 

 muscular contraction, becomes transformed into mechani- 

 cal work. The muscle is an animated machine^ which, 

 like the steam-engine, utilizes the heat in order to 

 produce work : in both cases there is necessarily an 

 equivalence between the heat which disappears, or is 

 consumed, and the external work achieved/ In con- 

 sideration of its origin, the energy manifested during 

 the contraction of the muscle is directly comparable 

 with the energy due to the elasticity of vapour when 

 this is the motor power at work, as in a steam-engine. 

 Chemical change combustion, in fact in each case, in 

 muscle and steam-engine alike, causes the liberation of 

 heat ; and in each case part of this liberated force is 

 capable of manifesting itself anew in the form of me- 

 chanical energy. It matters not whence the heat is 

 derived whether it comes from the decomposition of 

 the recently assimilated food-products in the blood 

 which circulates through the muscle, or whether it 

 proceeds from the liberated energy or sun-force that 



