METABOLISM OF MATTER AND ENERGY. 1017 



the more general applications of the law of the conservation of energy 

 in the animal body. Although there is no reason to doubt this the 

 experimental proof is a matter of great interest and importance. 



Experiments on the income and outgo of energy are comparatively 

 recent. They are very technical and necessitate costly and elaborate 

 apparatus. The results which may be obtained, however, are of extreme 

 importance and their application is widespread. The Connecticut Storrs 

 Experiment Station, in cooperation with this Office, is carrying on elab- 

 orate investigations on this matter and it is hoped the work will be 

 enlarged. 



THE RESPIRATORY QUOTIENT. 



In the respiratory experiments previously mentioned the total income 

 and outgo of carbon was determined. There is another class of respi- 

 ration experiments in which the total income and outgo is not the 

 factor under consideration, but rather the amount of carbon dioxid 

 excreted in a given time as compared with the amount of oxygen con- 

 sumed from the air. This ratio of carbon dioxid to oxygen is called 

 the respiratory quotient and experiments in which it is determined may 

 be conveniently called respiratory quotient experiments. For such 

 work special apparatus is needed. In some cases the subject is con- 

 fined in a chamber not unlike that of the other form of respiration 

 apparatus and the amount of oxygen consumed and carbon dioxid pro- 

 duced is measured. In other cases a tube is inserted in the trachea of 

 the animal. Through this tube all inspired and respired air must pass. 

 The tube is provided with suitable valves, and the incoming and out- 

 going air is measured, sampled, and analyzed. This device does not 

 cause any serious inconvenience to the animal experimented with. 

 Horses and dogs used in such experiments have lived in health for 

 many years after the operation was performed. In experiments with 

 man and sometimes with animals a mouthpiece or a mask over the 

 head is worn, which, by an arrangement of valves, permits the meas- 

 urement of the inspired and respired air. 



The respiratory quotient varies markedly under different experi- 

 mental conditions and is a delicate index of the processes going on in 

 the body. 



It has been found that protein, fat, and carbohydrates each give a 

 different respiratory quotient when burned in the body. It is therefore 

 possible to judge of the substances actually burned in the body by 

 variations in the respiratory quotient. Muscular work, either internal 

 or external, affects the respiratory quotient, since it increases the amount 

 of carbon metabolized, and hence the carbon dioxid excreted. From 

 the ratio the amount of work performed can be computed. Thus by 

 changes in the respiratory quotient it is possible to measure the energy 

 expended — for instance, in chewing and digesting different foods. 

 This affords a method for determining the actual value of different foods 



