SEC. 2. THE ENERGY OF THE BODY. 



The Income of Energy. 



527. Broadly speaking, the animal body is a machine for 

 converting potential into actual energy. The potential energy is 

 supplied by food ; this the metabolism of the body converts into 

 the actual energy of heat and mechanical labour. We have in 

 the present section to study what is known of the laws of this 

 conversion, and of the distribution of the energy set free. 



Neglecting all subsidiary and unimportant sources of energy, 

 we may say that the income of animal energy consists in the 

 oxidation of food into its waste products, viz. the oxidation of 

 proteids, fats and carbohydrates into urea, carbonic acid and water. 

 A principle laid down by the chemist teaches that the potential 

 energy of any body, considered in relation to any chemical change 

 which it may undergo, is the same when the final result is the 

 same, whether that result be gained at one leap or by a series of 

 steps ; that, for instance, the energy set free by the oxidation of 

 1 grm. of fat into carbonic acid and water is the same, whatever the 

 changes forwards or backwards which the fat undergoes before it 

 finally reaches the stage of carbonic acid and water ; and similarly, 

 that the energy available for the body in 1 grm. of dry proteid is 

 the energy given out by the complete combustion of that 1 grm., 

 less the energy given out by the complete combustion of that 

 quantity of urea to which the 1 grm. of proteid gives rise in the 

 body. Taking this as our guide we can readily calculate the 

 amount of potential energy contained in an average 24 hours' diet, 

 and thus obtain the average daily income of energy; for the 

 potential energy of most of the substances used as food has been 

 determined by direct calorimetric observations, and these have, by 

 improved methods, been brought to such a degree of accuracy that 

 they may, without risk of any great error, be taken as data for the 



