THE ENERGY OF THE METABOLISM 183 



than that actually produced as a result of his metabolism. 

 When the books had all been returned to the original level 

 the sum of the calories for the two periods would justly 

 represent the energy production of his body. One could 

 conceive of two calorimeters placed side by side, in one of 

 which a man might turn a crank operating a shaft which 

 should pass into the second chamber and there revolve a 

 wheel against the resistance of a brake. Most of his energy 

 would be registered by the first calorimeter, but a fair pro- 

 portion of it, standing for most of his muscular work, would 

 be apparent in the second. 



Direct and Indirect Calorimetry. We have said else- 

 where that the proof of the validity of the principle of the 

 conservation of energy for living things was a great achieve- 

 ment of the nineteenth century physiologists. The 

 method of this proof may now be outlined. We have just 

 seen that the total energy production of animals, including 

 man, may be satisfactorily measured in calorimeters, 

 provided that the amount of evaporation is known and 

 any changes of body temperature considered. This pro- 

 cedure is direct calorimetry. Now, of course, it is possible, 

 during the same period to collect the excreta and to deter- 

 mine the character and the amount of the metabolism 

 according to the principles explained in the last chapter. 

 Knowing the metabolism so many grams of protein, of 

 fat, and of glycogen destroyed we may credit to each of 

 these its respective fuel value and calculate the number of 

 calories which could theoretically result from just such 

 oxidation. We may then compare the energy as deter- 

 mined from the living organism and the energy which 

 should have been liberated in the formation of the measured 

 wastes. 



To make the matter plain, we may refer once more to the 

 case on which the numerical estimation of metabolism 

 was previously based. The subject was credited with a 

 metabolism of 100 grams of protein and 223.6 grams of fat. 

 (This, it may be recalled, is the usual assumption when 

 the day is one of fasting and does not closely follow carbo- 



