GENERAL METABOLISM. 627 



than an ordinary steam engine. We must admit, on the other hand, 

 that the way this was proved by Atwater's experiments is not altogether 

 beyond reproach. It is not an exact determination. We do not know 

 just how much effect the increased muscular work had upon the functions 

 of the other organs. At the same time it is probable that the values 

 obtained are not far from the truth. 



While the advance in technique and in methods results in more precise 

 investigations, still, on the other hand, we are often compelled to resort 

 to indirect methods with all their sources of error which we have so often 

 mentioned. This is true in many cases when we attempt to determine 

 the participation of nitrogen-free foodstuffs in metabolism. We can, it 

 is true, get some idea of this by comparing the volumes of inspired and 

 expired air. In normal breathing the volume of the expired air is always 

 greater than that of inspired air. This is due to the fact that the outer 

 air is warmed to the body temperature after it is inspired, and at the same 

 time it is almost completely saturated with water vapor. In order to 

 obtain actually comparable figures it is necessary to measure the two 

 volumes of air under precisely the same conditions. Both must be brought 

 to the same temperature and pressure; and, furthermore, they must be 

 dried. When this is done, it will be found that the volume of the expired 

 air is almost invariably smaller than that inspired. This is due to the 

 fact that in the combustion of the foodstuffs, the carbohydrates alone 

 yield a volume of carbon dioxide equal to that of the oxygen consumed, 

 while in the combustion of protein and fat this is not the case. In the case 

 of the latter, a part of the inspired oxygen is utilized for the formation of 

 water, sulphuric acid, and other substances. The amount of oxygen con- 

 sumed in this way does not appear in the volume of the expired air when 

 measured as above. In the combustion of carbon, one volume of carbon 

 dioxide gas is formed from one volume of oxygen. In this case the ratio 



PO 



x = 1. This relation of expired carbon dioxide to inspired oxygen is 

 U 2 



called the respiratory quotient. In the combustion of carbohydrates 

 this is 1. A diet in which protein predominates causes the quotient to 

 fall to about . 80, while if fat is chiefly concerned in the metabolism, the 

 value of the quotient falls to 0.70. According to the value of this re- 

 spiratory quotient, therefore, we can draw some conclusions regarding the 

 nature of the food upon which the subject experimented upon is working. 

 If, for example, a dog which has a good supply of glycogen stored up is made 

 to fast, then the high respiratory quotient, which is approximately 1, 

 shows that at the given moment the dog is maintaining its economy 

 chiefly by drawing upon its carbohydrate stores. When the value of the 

 quotient begins to fall, it shows that fat is being consumed, and finally it 

 will be compelled to utilize its own protein. Naturally, it is not, in general. 



