630 SALTS AS FOOD. [Book ii. 



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 peptone or albumose. A difficulty, apper- 

 taining to digestion, prevents any large substitution of peptone 

 for ordinary proteids, since as might be expected diarrhoea is 

 apt to be set up. 



§ 420. 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 metabo- 

 lism 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 im- 

 portant from a biological point of view than carbon or nitro- 

 gen ; it is as absolutely essential for the growth of a lowly 

 being like Penicillium as for man himself. We find it proba- 

 bly playing an important part as the conspicuous constituent of 

 lecithin and other complex fats belonging to the nervous sys- 

 tem, we find it prominent in the peculiar body nuclein, we find 

 it peculiarly associated with the proteids ; but we cannot ex- 

 plain 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 were withdrawn.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 ap- 

 pendages, 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 pos- 

 sible 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 system, is going wrong ; they suffer from 

 weakness, soon amounting to paralysis, and are often carried 

 off by convulsions. And more or less similar derangements of 

 nutrition follow the absence or a deficiency of individual salts. 

 During starvation these various salts continue to be discharged 

 from the body ; in some way or other they are carried along 



