PHYSIOLOGICAL 339 



work, or even digestion — than when it is merely maintaining a 

 constant temperature and performing the necessary work of breath- 

 ing, pumping the blood through the arteries, and so forth. When 

 the work of these French pioneers was followed nearly a century 

 later by the precise studies of Rubner and others, real proof was 

 obtained of what was even in these early experiments clearly 

 indicated, that in its chemical exchanges the animal body is a 

 machine, neither creating nor destroying energy, and obeying the 

 laws of thermodynamics. 



The three great classes of foods are carbohydrates, fats, and 

 proteins; and the latter are of special importance, inasmuch as they 

 serve to repair broken-down tissue and make good the inevitable 

 wastage, as well as for new growth. Also requisite for the mainten- 

 ance of life are certain inorganic salts, compounds of the rarer 

 necessary elements, besides vitamins and other accessories, which 

 are important by virtue of their chemical peculiarities, rather than 

 because they act as chemical sources of energy. The proteins not 

 only supply material for repair and for growth, they also serve to 

 furnish energy to the body by oxidations, just as the fats and 

 carbohydrates do. Not one of the three types of food is capable 

 of yielding quite as much energy as an equal weight of pure carbon 

 or hydrogen oxidised experimentally, for the reason that in these 

 compounds there is already a certain amount of oxygen. Consider, 

 for example, the case of the most typical and most important carbo- 

 hydrate, glucose, whose chemical formula is CeHjaOe. As there is 

 already enough oxygen to combine with all the hydrogen present 

 to form water, it follows that no energy can be derived from the 

 oxidation of hydrogen in the case of sugars. On the other hand, 

 each of the six carbon atoms will combine with two atoms (one 

 molecule) of oxygen to form one molecule of carbon dioxide, and 

 this reaction will yield energy. For every molecule of oxygen used 

 up when glucose or any other carbohydrate is burnt in the body, 

 one molecule of carbon dioxide is produced; and since equal volumes 

 of all gases under like conditions contain equal numbers of molecules, 

 the ratio of carbon dioxide breathed out to oxygen consumed will 

 be I : I, by volume, when carbohydrates alone are being oxidised. 



This ratio, the Respiratory Quotient, is of great importance as an 

 index of the processes going on within the body. For if fats or 

 proteins are being oxidised, the respiratory quotient will fall, 

 perhaps to 7 : 10; the reason being that these substances contain 

 proportionately less oxygen than carbohydrates, so that more 

 oxygen must be breathed in if all the hydrogen atoms of the food- 

 material are to be oxidised to water. In fact, while the carbohydrates, 

 with such formulae as CeHjaOg and C12H22O11, may be regarded as 

 carbon plus water, the fats have the composition C57HJJ0O6, or 

 something of the sort, that is to say, carbon plus water, plus a hrge 



