THE STATISTICS OF NUTRITION. 491 



the place of circulatiug but not of tissue proteid. What is the cause of this 

 difference we cannot at present say. 



439. Peptone as food. Since proteids are at least largely, as we have 

 seen ( 262), converted into and absorbed as peptone, and since, as we have 

 also seen, the peptone appears during the very act of absorption to be recon- 

 verted 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. Neverthe- 

 less, some observers have maintained with regard to both peptones and the 

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

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

 goes to show 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 diarrhoea is apt to be set up. 



440, The effects of salts as food. All food contains, besides the sub- 

 stances possessing potential energy, which we have just studied, certain saline 

 matters, organic and inorganic, having in themselves little or no such poten- 

 tial 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 im- 

 portant, 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 con- 

 spicuous 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 con- 

 stituent 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 kreatin of the epidermis and its appendages, hairs, etc., 

 sulphur is probably undergoing excretion, though its presence in kreatin may 

 have to do with the peculiar physical characters of corneous epithelium. 



AVe 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 

 showing 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 indi- 

 vidual salts. During starvation these various salts continue to be discharged 

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

 stream, and their presence is in some way essential to the various metabolic 

 processes ; hence, they need to be always present in daily food. In what 

 way it is that they thus direct metabolism we do not know ; we are aware 

 that the properties and reactions of various proteid substances are closely 

 dependent on the presence of certain salts, but beyond this we know very 

 little. The inorganic salts are those the nutritive value of which has been 

 chiefly studied by experiment, but we have reason to believe that the organic 

 salts, or extractives, which are present in greater or less quantity in all food 

 of both vegetable and animal origin, are no less essential to the proper meta- 



