THE ENERGY REQUIREMENTS OF THE BODY 425 



amount of heat is measured in terms of calories, or more often in large calories. 

 The work energy may be expressed in gram-centimeters or in kilogrammeters. 

 The heat-unit calories may be transferred into the work-unit gramcenti- 

 meters by multiplying by .042, and the converse. 



The source of the heat and work energy which is produced in the body is 

 from the metabolic changes of the tissues, the chief part of which is of the 

 nature of oxidation, since it may be supposed that the oxygen of the atmos- 

 phere taken into the system is ultimately combined with carbon and hydrogen. 

 Any change, indeed, which occurs in the protoplasm of the tissues, resulting 

 in an exhibition of its function, is attended by the evolution of heat and the 

 formation of carbon dioxide and water. The more active the changes the 

 greater is the heat produced. But, in order that the protoplasm may per- 

 form its function, the waste of its own destructive metabolism must be re- 

 paired by the due supply of food material to be built up in some way into the 

 protoplasmic molecule. Food is therefore necessary for the production of 

 heat. In the tissues, as we have several times remarked, two processes are 

 continually going on : the building up of the protoplasm from the food, anab- 

 olism, which is not accompanied by the evolution of heat, and the oxidation 

 of the protoplastic materials, catabolism, resulting in the production of energy, 

 by which heat is set free. It is not necessary to assume that the combustion 

 processes, indeed, are as simple as the bare statement of the fact might seem 

 to indicate. But complicated as the various stages may be, the ultimate re- 

 sult is as simple as in ordinary combustion outside the body, and the prod- 

 ucts are the same. 



This theory, 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, 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 406; an amount capable 

 of maintaining the temperature of the body at from 36.8 to 38.7 C., 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 gradually. 

 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, result- 

 ing 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 proteid food it is a little different, 



