THE ENERGY REQUIREMENTS OF THE BODY 477 



ous stages may be, the ultimate result is as simple as in ordinary combustion 

 outside the body, and the products are the same. 



This view, that the maintenance of the temperature of the living body 

 depends on continual chemical change, chiefly by oxidation of combustible 

 materials in the tissues or by the tissues, has long been established. The 

 quantity of carbon and hydrogen supplied as food, chiefly in the form of 

 carbohydrates and fats which, in a given time, unites in the body with 

 oxygen, is sufficient to account for the amount of heat generated in the 

 animal within the same period, page 454. This amount is capable of 

 maintaining the temperature of the body at from 36.8 to 38.7C., not- 

 withstanding a large loss by radiation and evaporation. This estimation 

 depends upon the chemical axiom that when a body undergoes a chemical 

 change the amount of energy set free is the same, supposing the resulting 

 products are the same, whether the change takes place suddenly or gradu- 

 ally. If a certain number of grams of different substances are introduced 

 as food, and if they undergo complete oxidation, the amount of kinetic 

 energy, as shown in the amount of heat and mechanical work, is the same 

 as would be developed if the same bodies were completely oxidized outside 

 the body. If one gram of fat be taken into the body and is completely 

 oxidized, resulting in the production of a definite amount of carbon dioxide 

 and water, it may be supposed to have produced the same amount of heat 

 as it would have produced outside the body. In the case of protein food 

 it is a little different, since it is never completely oxidized within the body, 

 but may be supposed to give rise to a definite amount of urea and other 

 lower nitrogenous compounds not completely oxidized in the body. In 

 this case the gram of protein may be considered to liberate the same 

 amount of heat as the protein would outside the body minus the amount 

 which would be obtained from the complete oxidation of the resulting 

 urea, etc. 



The actual amount of heat produced per diem has been experimentally 

 ascertained in the case of man and animals by the aid of an apparatus, the 

 calorimeter. An animal is enclosed in a metal cage completely contained 

 in a second cage containing water. Air is let into and out of the inner box 

 by means of metal tubes so arranged that the inlet tubes maintain a con- 

 stant temperature and the outlet tubes pass through water between the 

 two chambers. The heat given out by the animal warms the water in 

 the outside box, and may be estimated by the rise of its temperature, the 

 amount of which is known. At the same time the carbon dioxide output 

 is measured. 



The rate of human metabolism has come to be of great importance in 

 clinical diagnosis of nutritional states. The direct methods are too in- 

 volved for rapid testing. Indirect determinations are quite accurate 

 enough for such purposes. The indirect method rests on measurement 



