WATER AND SODIUM CHLORIDE 39 



hay exclusively for three and a half months. At the same time a 

 similar batch of animals was fed exclusively upon cow's milk. Now 

 these two diets contained very different relative amounts of sodium 

 and potassium, hay being much richer in potassium than in sodium, 

 and milk richer in sodium than in potassium. Yet at the end of the 

 period the composition of the blood obtained from the two groups, 

 as regards sodium and potassium, was identical. The tissues, not only 

 the epithelium of the kidney but that of the intestine as well, actively 

 choose the constituents which they will reject or absorb respectively. 

 In just the same way a plant, living in water rich in sodium and poor 

 in potassium, will nevertheless pick the potassium out and build it 

 up into tissues which are rich in potassium and poor in sodium. But 

 this power of selection is limited, and in extreme cases, as, for example, 

 a diet so rich in potassium as potatoes, some aid is required, and sodium 

 and chlorine in the form of common salt must be added to the dietary. 



From the standpoint of physical chemistry it is of course evident 

 that selective absorption of mineral salts by the epithelium of the 

 intestine or their selective elimination by the kidneys must involve 

 the performance of work; the expenditure of energy. For the osmotic 

 pressures of the various salts in the solutions bathing the cells would 

 tend to drive them into the absorbing or excreting tissues in pro- 

 portion to their concentration and if, on the contrary, they appear on 

 the other side of these epithelial tissues in emphatic disproportion to 

 their original concentrations, the process of assimilation or excretion 

 must have involved the overcoming of the forces of Osmotic Pressure. 

 The energy necessary to achieve this can only be derived from the 

 combustion of other foodstuffs or constituents of tissues which are thus 

 robbed of the supplies available for carrying on the other activities 

 of the body. Selective absorption or excretion implies work, therefore, 

 and anything which relieves the tissues in any measure of the necessity 

 of exercising selection sets free a certain number of heat-units for other 

 uses or, in other words, improves the utilization of other foodstuffs. 

 The gratification and frequent improvement in nutrition which 

 accompanies the administration of salt to herbivorous animals may 

 thus originate in relief of the tissues from the strain and burden of 

 selection and the liberation of foodstuffs for the maintenance of other 

 tissue-activities which is in effect, equivalent to the addition of a certain 

 amount of food to the accustomed dietary. The beneficial effects of 

 salt may therefore, and in the long run, reside not so much in the 

 actual sodium and chlorine administered as in the additional carbo- 

 hydrate; fat, or protein which is thus rendered available for the main- 

 tenance and upbuilding of the body. 



It is probably for some such reason as this that the total mineral- 

 requirements of the body vary exceedingly with the dietary upon which 

 an animal is subsisting.' Especially is this the case when the require- 

 ment on a normal mixed diet is contrasted with that which obtains 

 when the diet is limited in such a way as to provide only those proteins 



