CHAP, v.] NUTRITION. 799 



of tissue proteid. What is the cause of this difference, we cannot 

 at present say. 



525. Peptone OS 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 as 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 gelatine, 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 peptone or albumose. A 

 difficulty, appertaining to digestion, prevents any large substitution 

 of peptone for ordinary proteids, since as might be expected diar- 

 rhoea is apt to be set up. 



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

 tlie 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 

 element phosphorus seems no less important from a biological 

 point of view than carbon or nitrogen ; it is as absolutely 

 essential for the growth of a lowly being like Penicillium as for 

 man himself. We find it probably playing an important part as 

 the conspicuous constituent of lecithin and other complex fats 

 belonging to the nervous system, we find it prominent in the 

 peculiar body nuclein, we find it peculiarly associated with the 

 proteids ; but we cannot explain its role. The element sulphur, 

 again, is only second to phosphorus, and we find it as a constituent 

 of nearly all proteids ; but we cannot foretell the exact changes 

 which would take place in the economy if all the sulphur of the 

 food were withdrawn. In the keratin of the epidermis and its 

 appendages, hairs &c., it is probably undergoing excretion, though 

 its presence in this body may have to do with the peculiar 

 physical characters of corneous epithelium. 



We know that the various saline matters are essential to 

 health, that when they are not present in proper proportions 

 nutrition is affected. Dogs fed on food, freed as much as possible 

 from all saline matters, but otherwise abundant, with a proper 

 proportion of the food stuffs, soon exhibit symptoms shewing that 

 the metabolism of their tissues, especially of their central nervous 



