CHEMISTRY OF DIGESTION AND NUTRITION. 295 



It has been shown, for example, that if egg-albumin is completely deprived 

 of its ash, it is no longer soluble in water. We may assume that the original 

 synthesis of the organic and inorganic constituents is made in the plant king- 

 dom, and that, in its own way, the inorganic constituent of the molecule is as 

 necessary to the proper nutrition of the animal tissues as is the organic. One 

 salt (NaCl) is consumed by many animals, including man, in excess of the 

 amount unconsciously ingested with the food. Bunge points out that purely 

 carnivorous animals are not known to crave this salt, while the herbivora 

 with some exceptions for example, the rabbit take it at times largely in ex- 

 cess. The need of salt on the part of these animals is well illustrated among 

 the wild forms by the eagerness with which they visit salt-licks. Bunge 

 advances an ingenious theory to account for the difference in regard to the use 

 of salt between the herbivora and the carnivora. He points out that in plant 

 food there is a relatively large excess of potassium salts. When these salts 

 enter the liquids of the body they react with the NaCl present and a mutual 

 decomposition ensues, with the formation of KC1 and the sodium salt of the 

 acid formerly combined with the potassium, and the new salts thus formed are 

 eliminated by the kidneys as soon as they accumulate beyond the normal limit. 

 In this way the normal proportion of NaCl in the tissues and the body-fluids 

 is lowered and a craving for the salt is produced. Buuge states that it has been 

 shown among men that vegetarians habitually consume more salt than those 

 who are accustomed to eat meats. The salts of calcium and of iron have also 

 a special importance which needs a word of reference. The particular import- 

 ance of the iron salts lies in their relation to haemoglobin. The continual 

 formation of new red blood-corpuscles in the body requires a supply of iron 

 salts for the synthesis of the haemoglobin, and, although there is a probability 

 (see p. 263) that the iron compound of the disintegrating corpuscles is again 

 used in part for this purpose, we must suppose that the body requires addi- 

 tional iron in the food from time to time to take the place of that which is 

 undoubtedly lost in the excretions. It has been shown that iron is contained 

 in animal and vegetable foods in the form of an organic compound, and the 

 evidence at hand goes to show that only when it is so combined can the iron 

 be absorbed readily and utilized in the body, while the efficacy of the inor- 

 ganic salts of iron as furnishing directly a material for the production of haemo- 

 globin is, to say the least, open to doubt. Bunge isolated from the yolk of 

 eggs an iron-containing nuclein which he calls hcematogen, because in the 

 developing hen's egg it is the only source from which the iron required 

 for the production of haemoglobin can be obtained. It is possible that sim- 

 ilar compounds occur in other articles of food. Most of the iron taken with 

 food, however, including that present in the haemoglobin of meats, passes 

 out in the feces unabsorbed. It is probable that there is an actual excre- 

 tion of iron from the body, and, so far as known, this excretion is effected 

 in small part through the urine, but mainly through the walls of the intes- 

 tine, the iron being eliminated finally in the feces. The large proportion of 

 calcium salts found in the skeleton implies a special need of these salts in 



