506 THE RESPIRATORY EXCHANGE 



effect upon the intake of oxygen when they were injected into 

 the blood. In the case of a horse, Zuntz and Hagemann calculated 

 that nearly half the available energy of the hay would be spent 

 in the work of mastication and digestion. These views are sup- 

 ported by Lowy, who determined the respiratory exchange of 

 fasting men before and after a dose of sodium sulphate or a 

 draught of cold water, whereby the alimentary canal was stimu- 

 lated. The respiratory exchange was increased by 10 or 20 per 

 cent., but was unaltered when sodium chloride and sodium- 

 bicarbonate were given instead of sodium sulphate. Lowy 

 suggests that the therapeutic value of the waters of Carlsbad and 

 Marienbad may be partly due to this stimulating effect of sodium 

 sulphate. It is probable, however, that Zuntz has overestimated 

 the part played by the activity of the alimentary canal, for experi- 

 ments by Magnus-Levy, Bleibtreu, Koraen, Pembrey, and Spriggs( 13 ) 

 show that the increased respiratory exchange is determined not 

 so much by the digestibility of the food as its effect upon the meta- 

 bolism of the body. This is even granted by Zuntz himself in 

 order to explain the marked effect of maize upon the respiratory 

 exchange of a horse. 



The effect of food depends upon its quality and quantity. 

 Fat appears to produce only a very slight increase in the respiratory 

 exchange, less than 10 per cent. ; proteid, a rise of 30 to 50 per 

 cent, of the amount observed in the fasting animal. A meal rich 

 in carbohydrates causes a much more marked effect upon the 

 output of carbon dioxide than upon the intake of oxygen ; the 

 former may in the case of rats rise to 97 per cent., the latter to 

 35 per cent, above the values obtained in the fasting condition. 

 The increase is well marked within an hour of the meal, and 

 steadily increases during the next two or three hours. Although 

 the output of carbon dioxide is 14 to 97 per cent, higher than 

 the minimal discharge during hunger, the oxygen absorbed is 

 increased to a less extent, and thus the respiratory quotient may 

 rise to 1*17, and remain as high as 1'14 for a period of twenty hours. 

 The explanation is to be found in the conversion of carbohydrates 

 into fats, a chemical change accompanied by the liberation of 

 carbon dioxide. These high quotients are only observed during 

 the deposition of fat, and have already been discussed in an earlier 

 part of this article. 



