212 Metabolism of Healthy Man. 



mediate steps between the absorption of oxygen and the excretion of carbon 

 dioxide resulting from the oxidation of any material in the body. That these 

 steps follow with lightning-like rapidity, one after the other, is hardly conceiv- 

 able. On the other hand, it seems clearly proved that in an experiment lasting 

 for 24 hours the delay in the excretion of carbon dioxide following the absorp- 

 tion of oxygen or the delay in the excretion of nitrogen following the katabolism 

 of protein is either so uniformly timed at the beginning and end of each experi- 

 ment as to compensate, or else the possible errors involved in assuming an im- 

 mediate excretion are not greater than those involved in the determination of 

 the total 24-hour amounts. Experiments are greatly needed for the comparison 

 of indirect and direct calorimetry, namely, the determination of the katabolized 

 materials for short periods and the control of these determinations by an esti- 

 mation of the heat production during these periods, calculating from the katab- 

 olized materials the energy that they would produce when oxidized. By this 

 means and this means only will it be possible for us to throw light upon the 

 rapidity of oxidation and excretion of material and the coincidence in the pro- 

 duction of heat, the absorption of oxygen, and the excretion of carbon dioxide. 



Until this comparison has been made and definite results obtained, it is neces- 

 sary, particularly in experiments of short duration, to interpret with some 

 reserve the katabolism indicated by the respiratory quotient. In the experi- 

 ments here reported, the nitrogen excretion in the urine was not determined for 

 reasons previously stated; * consequently, it is not practicable for us to compute 

 indirectly the katabolism and apportion it between the protein, fat, and carbo- 

 hydrate, and from this apportionment of materials katabolized, compute the 

 total heat production. The respiratory quotients were determined in most 

 instances, and since the respiratory quotients have a general interest as indi- 

 cating the nature of the material katabolized by man under ordinary conditions, 

 they have been computed and are here presented. 



Respiratory Quotients in Metabolism Experiments. 

 During sleep. The difficulties incidental to determining the oxygen directly, 

 especially in experiments of short duration, have already been pointed out. 

 Unless the subject is under essentially the same conditions of muscular activity 

 at the beginning and end of each period, the determinations of oxygen are liable 

 to be vitiated. It is also necessary in order to determine the oxygen to know 

 with considerable accuracy the exact temperature of the air inside of the respira- 

 tion chamber. This temperature is determined by means of resistance ther- 

 mometers whereby an average temperature is obtained. If the subject is lying 

 covered with a blanket at the beginning of a period and is sitting in a chair at 

 the end of the period, there are obviously very different thermometric condi- 

 tions inside the chamber at the beginning and end of the period, and, as a result, 

 there may be a considerable error in the exact measurement of oxygen. On the 

 other hand, we find that the best conditions for measurements of temperature 



1 See p. 11. 



