290 THE HUMAN BODJ. 



mations does not first take the form of heat; though some 

 of it does. This, again, does not affect the general prin- 

 ciple: the source of energy is essentially the same in both 

 cases; it is merely the form which it takes that is dif- 

 ferent. In a galvanic cell energy is liberated during the 

 union of zinc and sulphuric acid, and we may so arrange 

 matters as to get this energy as heat; but on the other 

 hand we may lead much of it off, as a galvanic current, 

 and use it to drive a magneto-electric machine before it 

 has taken the form of heat at all. In fact, that heat may 

 be used to do mechanical work we must reduce some of it 

 to a lower temperature: an engine needs a condenser of 

 some kind as well as a furnace; and, other things being 

 equal, the cooler the condenser the greater the proportion 

 of the whole heat liberated in the furnace which can be 

 used to do work. Now in a muscle there is no condenser; 

 its temperature is uniform throughout. So when it contracts 

 and lifts a weight, the energy employed must be liberated 

 in some other form than heat some form which the muscu- 

 lar fibre can use without a condenser. 



Summary. The living Body is continually losing mat- 

 ter and expending energy. So long as we regard it as 

 working by virtue of some vital force, the power of gener- 

 ating which it has inherited, the waste is difficult to 

 account for, since it is far more than we can imagine as due 

 merely to wear and tear of the working parts. When, 

 however, we consider the nature of the income of the Body, 

 and of its expenditure, from a chemico-physical point of 

 view, we get the clue to the puzzle. The Body does not 

 waste because it works but works because it wastes. The 

 working power is obtained by chemical changes occurring 

 in it, associated with the liberation of energy which the 

 living cells utilize; and the products of these chemical 

 changes, being no longer available as sources of energy, are 

 passed out. The chemical changes concerned are mainly 

 the breaking down of complex and unstable chemical com- 

 pounds into simpler and more stable ones, with concomi- 

 tant oxidation. Accordingly the material losses of the 



