Chap, v.] NUTRITION. 629 



portion of the oxygen consumed reappears in the carbonic acid 

 of the egesta than in carnivora, in which animals, living chiefly 

 on proteids and fats, more of it leaves the body combined with 

 hydrogen to form water. This relation of the oxygen to the 

 carbonic acid is often expressed as the quotient of the volume 

 of the carbonic acid expired divided by the volume of the 



oxygen consumed, the 'respiratory quotient,' — , which is in 



herbivora about -9 and in carnivora about -6 or -7. When a 

 herbivorous animal starves, it feeds on its own fat, and under 

 these circumstances the respiratory quotient falls to the car- 

 nivorous standard ; and indeed many circumstances affect this 

 respiratory quotient. The carbohydrates are notably more 

 digestible than the fats, but on the other hand the fats contain 

 more potential energy in a given weight. As to the nutritive 

 difference between starch and sugar, we know nothing very 

 definite ; it has been thought however that cane-sugar is rather 

 more fattening than starch. 



§ 418. The Effects of Gelatin as Food. It is a matter of 

 common experience that gelatin will not supply the place of 

 proteids as a constituent of food. Animals fed on gelatin 

 together with fat or carbohydrates die very much in the same 

 way as when they are fed on non-nitrogenous material alone. 

 • Nevertheless it would appear, as might be expected, that the 

 presence of gelatin in food is not without effect. Thus nitro- 

 genous equilibrium is established at a lower level of real proteid 

 food when gelatin is added. In a dog, moreover, fed on a diet 

 of gelatin and fat, the excess of nitrogen in the excreta over 

 that in the ingesta is less than when the same dog is fed on a 

 diet of fat alone ; that is to say, the gelatin has sheltered from 

 metabolism some proteid constituents of the body; and the 

 consumption of fat seems also to be lessened by the presence of 

 gelatin. These facts become intelligible if we suppose that 

 gelatin is rapidly split up into a urea and a fat moiety, in the 

 same way that we have seen a certain quantity of proteid ma- 

 terial to be. It is this direct destructive .metabolism of proteid 

 matter which gelatin can take up; it seems however unable to 

 imitate the other function of proteid matter, and to take part 

 in the formation of living substance; or in the phraseology of a 

 preceding paragraph (§ 416), it can take the place of circulat- 

 ing but not of tissue proteid. What is the cause of this differ- 

 ence, we cannot at present say. 



§ 419. Peptone as Food. Since proteids are at least largely, 

 as we have seen (§ 250), converted into and absorbed as pep- 

 tone, and since as we have also seen the peptone appears during 

 the very act of absorption to be reconverted into some other 

 form of proteid matter, possibly serum-albumin, it might seem 

 natural to suppose that peptone given as food would as far as 



