CHAP, v.] NUTRITION. 835 



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 nitrogenous equilibrium is established 

 at a lower level of real proteid food when gelatin is added ; indeed, 

 for a short time at least, a large portion of the proteid of a diet 

 may be replaced by gelatin without materially changing the nitro- 

 genous output. 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 in- 

 telligible 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 material 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 ( 522), it can take the 

 place of circulating but not of tissue proteid. What is the cause 

 of this difference we cannot at present say. 



525. Peptones as Food. Since proteids are at least largely, as 

 we have seen ( 309), converted into and absorbed as peptone, 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 so far as metabolism is concerned 

 play the same part as other proteids. Nevertheless, some observers 

 have maintained with regard to both peptones and the allied 

 albumoses that, like gelatin, these bodies " can take the place of 

 circulating but not of tissue proteid." On the whole, however, the 

 evidence goes to shew that animals can ' lay on flesh ' when the 

 proteid in their food consists entirely of peptones or albumoses. A 

 difficulty, appertaining to digestion, prevents any large substitution 

 of these bodies for ordinary proteids, since as might be expected 

 diarrhoea is apt to be set up. 



526. The Effects of Salts as Food. All food contains, besides 

 the substances possessing potential energy, which we have just 

 studied, certain saline matters, organic and inorganic, having in 

 themselves little or no such potential energy, but yet either 

 absolutely necessary or highly beneficial to the body. These must 

 have important functions in directing the metabolism of the body : 

 the striking distribution of them in the tissues, the preponderance 

 of sodium and chlorides in blood-serum and of potassium and 

 phosphates in the red corpuscles for instance, must have some 

 meaning ; but at present we are in the dark concerning it. The 



