Chloride of sodium likewise enjoys the distinction of being the only inorganic substance which it is necessary to add to our diet. This is all the more remarkable because both animal and vegetable foods contain considerable proportions both of sodium and chlorine, but the explanation is to be found in the decided preponderance of potash in vegetable foods as compared with the content of sodium. We have already dealt with this subject fully in Chapter II (Theories Of Metabolism - Continued). in connection with Bunge's theory, but it may be profitable to add a few more details just at this point. It is notable that only the true herbivora and not the carnivora crave for salt, despite the fact that the chloride of sodium content in the food of both classes of animals is practically the same. The only factor in operation is the three or four times greater quantity of potash contained in the food of the herbivora, producing a double decomposition with the chloride of sodium in the blood, so that to preserve the chemical composition of the blood, the compulsory excretion alike of the chloride of potassium and the newly formed sodium salt takes place, thus depriving the serum of its necessary sodium chloride.

It may be taken for granted that this theory is correct, despite the fact that rabbits and hares live on foods rich in potash without craving for salt, and that cows and sheep can remain in perfect health on a similar diet without any addition of common salt.

It has been suggested that although the blood serum of carnivora and herbivora alike contains similar quantities of soda and potash, viz., .43 per cent. of soda and .026 per cent, of potash, the fact that the red corpuscles of certain classes of herbivora contain considerable amounts of soda is a sufficient explanation of this craving for salt. On the other hand, the red corpuscles of the carnivora also contain larger amounts of soda than those of the horse, cow, rabbit, etc, which subsist on foods rich in potash, without craving for salt. It is a notable fact that in the milk of the carnivora soda and potash are almost equally represented, whereas in the milk of the herbivora, as in human milk, the potash is to be found in greater proportion, so that from the moment of birth their respective organisms become accustomed to a fixed relation between the amounts of potassium and sodium. Hence the carnivora, eating the entire animal, consume almost equal proportions of the salts, whereas the herbivora and man, subsisting on many-foods with a preponderance of potassium, adjust their organism to this relationship. There is no doubt that some automatic method must be in existence for regulating the soda and potash content of the blood, as well as doubtless of its other equally essential salts.