452 HANDBOOK OF PHYSIOLOGY. 



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

 same if the same bodies are completely oxidized outside the body; so 

 that if 1 gramme of fat be taken into the body and the oxidation 

 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 proteidfood 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, not a completely oxidized body. In this case the 

 gramme of proteid may be considered to perform the same amount of 

 heat as the proteid would outside the body minus the amount which 

 would be obtained from the complete oxidation of the resulting urea. 



The actual amount of heat produced per diem has been experimentally 

 ascertained in the case of small animals by the aid of an apparatus 

 called a Calorimeter. The animal is inclosed in a metal box com- 

 pletely contained in a second box containing water, and air is led into 

 and out of the inner box by means of metal tubes ; the one through which 

 the air is led out of the chamber has several coils in it. 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. 



The amount of heat produced and of energy in the form of mechanical 

 work set free in a given time arise from the oxidation of the substances 

 taken in as food in so far as they are oxidized. In order that there may 

 be correct data to assist in the consideration of the subject, the amount 

 of heat evolved by the oxidation of various food-stuffs has been carefully 

 measured. The results may be set down in terms of gramme-calories 

 (Ca), a calorie being the heat unit, and meaning the amount of heat 

 required to raise 1 gramme of water 1 degree C., or, more strictly, from 

 15 C. to 16 C.* The number of gramme-calories which 1 gramme 

 of the following substances equals will be seen in the annexed table. 



Hydrogen . 3450 Fat . . 9000 Urea . 2200 



Carbon . . 8100 Carbohydrate 4000 



Proteid . 50005500 

 1 gramme of proteid giving rise to i gramme of urea. 



The relation between the income and expenditure of the body has 

 been already considered in detail in the preceding chapter. We may 

 now turn to the question of the chief heat-producing tissues. 



Heat-producing Tissues. (1.) The Muscles. As the muscles 

 form so large apart of the body, and as in them metabolism is particularly 

 active, it is only reasonable to consider the muscular as the chief heat- 



* Sometimes the term kilogramme-calorie is used ; one kilogramme-calorie 

 being equal to 1000 gramme-calories. 



