UTILISATION OF NUTRIENTS 49 



When the real available energy or availability of 

 the digested material is to be given, then the heat 

 value of the waste products must be deducted from 

 the total heat value contained in the digested 

 material. Thus from 100 grams of starch it has 

 been found by experiment on the ox that 3-17 grams 

 of marsh gas are formed, 1 gram of which has the 

 high heat value of 13,344 ca l- When this is calcu- 

 lated on 1 gram of starch it is seen that there is a 

 loss of 422 cal., so that the net heat value is 4183 

 cal. - 422 cal. = 3761 cal. or 3761 Cal. instead of 

 4-183 Cal. The available energy of 1 gram of 

 starch is then only 89-90%. 



The investigation of the energy metabolism can 

 be made in two ways, either direct or indirect. In 

 the first case the heat which is given off from the 

 animal is measured directly. For this purpose 

 special forms of apparatus — the so-called respira- 

 tion calorimeters are necessary. The commonest 

 form is built upon the principle of an ordinary air 

 thermometer, the bulb of which encloses the animal. 

 These calorimeters usually serve at the same time 

 as a respiration chamber. In the second case 

 where the determination is an indirect one all the 

 income and outgo of the animal in food, faeces, 

 urine, and the products of respiration must be 

 determined, as was done in the experiments on 

 storage of flesh and fat (pp. 44-47). In addition, 

 the heat values of all materials eaten or excreted 



