478 PKACTICAL PHYSIOLOGY 



Having estimated what the total excretion of carbon is, we 

 must now ascertain how much of it comes from proteid. To 

 do this multiply the total amount of nitrogen excreted by 3-3 

 (since proteids contain approximately 52*8 of carbon, and 16 of 

 nitrogen). If this amount of carbon be deducted from the total 

 amount excreted, the remainder corresponds to the carbon derived 

 from the combustion of fat and carbohydrate. As to which of these 

 two bodies it is from which the carbon really comes, we have no means 

 of telling definitely, but since there is very much more fat than carbo- 

 hydrate in the tissues we usually reckon it as fat. Each gramme of 

 carbon corresponds to 1'3 grammes of fat (because fat contains 76-5 

 grammes carbon). 



3. The Amount of Energy given out by the Body. The energy is 

 liberated in the body partly as heat, and partly as muscular work. 

 The amount actually lost as heat may be determined by placing 

 the animal in a respiration calorimeter, but it is impossible to estimate 

 directly, with anything like accuracy, the amount lost as mechanical 

 work. 



There are, however, certain indirect methods by which the total 

 amount of energy liberated may be determined, and these are as 

 follows : (a) By comparing the amount of food stuffs taken in with the 

 amount which reappears in the excreta, we can find out how much 

 of each food stuff has actually undergone metabolism in the tissues. 

 It is now quite easy to find how much energy this corresponds to, by 

 multiplying the amount of each food stuff metabolised by its caloric value. 

 (Where the diet contains both fat and carbohydrate, and where an 

 accurate balance of intake and output of carbon does not exist, we must 

 reckon the excess or deficit as fat, since there is much evidence to show 

 that the amount of carbohydrate in the body remains pretty constant.) 



(b) The extent of oxidation in the tissues is determined, not by the 

 amount of oxygen inspired, 1 but by the activity of the tissues. We 

 can, therefore, employ the amount of oxygen absorbed by the tissues as an 

 index of the amount of energy liberated in them. In order to do this, 

 however, it is necessary to remember that the amount of energy 

 liberated, when different food stuffs are burnt, is not the same ; thus 

 100 gr. of oxygen are necessary for the combustion of 35 gr. of 

 fat, the amount of energy hereby liberated amounting to 325 calories 



1 This statement may be true for slight variations in the amount of oxygen 

 supplied, but where the partial pressure of oxygen is increased to one atmosphere 

 a very marked depression in the output of C0 2 results. These experiments were 

 carried out on mice, the determinations being made by the gravimetric method 

 (Hill and Macleod). 



