144 THE HUMAN MOTOR 



(a) The Method of Nutritive Equilibrium. Since, as stated in 

 the preceding paragraph, the average composition of the ration 

 is approximately constant, we have only to adjust the amount 

 thereof until the weight of the bod}' remains constant. The 

 calori^c power of such a lation expresses the energetic expendi- 

 ture for twenty-four hours. But, as has been said, it is very 

 important that the temperature of the external environment 

 should be approximately constant. The subject must be in repose 

 if it is desired to ascertain his "static expenditure" or must 

 always perform the same work, if it is desired to know his 

 " dynamic expenditure." 



Hence, with this method, exceptional care is required to obtain 

 correct results. It is, however, useful if approximate results 

 only are required. 



(b] The Oxvgen Method. The preceding method determines 

 the quantity and the quality of the combustible. By this method 

 we measure the quantity of the combustion supporting gas, that 

 is to say, the volume of oxygen used by the organism. This is 

 very simple; foi by passing the gases expired by the lungs 

 through a " spirometer," the volume of air passing through the 

 organism in a certain period of time can be obtained. The 

 analysis of the gases expired gives their content of oxygen ; 

 and as the outside air contains, normally, oxygen to the amount 

 of 21% in volume, the difference shows the impoverishment of 

 the air in its passage throiigh the organism. The total volume 

 of oxygen consumed is deduced from the reading of the spiro- 

 meter. 



An easv method ( l ) consists in breathing through the mouth 



by means of Chauveau's valve (fig. 

 7" 130) ; the air is inspired at I and 

 expired at E, whence it goes into a 

 F, O 130. little receptacle before making its 



Chauveau's valve. exit through the meter. From this 



little receptacle one can take as small a sample as one likes, and 

 analyse it after the experiment (see Technics, 242, and fig. 131). 



A male adult consumes, in repose, about 20 litres of oxygen 

 measured at 0C and at a normal pressure of 760 millimetres, 

 The expenditure of oxygen is greatly increased by labour. 



We have next to determine the ratio between the amount of 

 energy actually expended and the corresponding volume of 

 oxygen absorbed. The energy is, no doubt, proportional to the 

 oxygen, and depends on the nature of the combustible : fats, 

 carbo-hydrates, or proteids, and their proportions in the ration. 

 Experiment has given for mixed alimentation a value of 4-90 



( J ) Jules Amar, Journal de Physiologic, March, 1911, p. 212). 



