362 LECTURE XVI. 



common salt we owe to G. von Bunge. 1 Bunge pointed out in the first 

 place that among animals only the true herbivora, and never the carnivora, 

 crave salt. This fact is familiar to hunters. They know that wild her- 

 bivora ruminants and solidungulates frequent the salt licks. Now 

 the amount of sodium chloride which these animals obtain in their food 

 is about the same per unit of the animal's weight as that obtained by the 

 carnivora in its diet. It is hardly to be said, therefore, that there is a 

 lack of sodium chloride unless we assume that this salt plays a particular 

 part in the organism of the herbivora, an assumption which, in the light 

 of recent investigations concerning the action of the individual ions, no 

 longer seems so improbable. Bunge has shown, however, that vegetable 

 food differs from animal food by the amount of potash which the former 

 contains. The herbivora obtain three or four times as much 'potassium 

 salt in the food as the carnivora do. All vegetables, especially potatoes, 

 clover, and meadow hay, contain large amounts of potash. We know of 

 but very few land plants which, like the varieties of Chenopodium and 

 Atriplex, contain more sodium chloride than potassium salt. It is easy 

 to account for the high potash content of plants by the distribution of the 

 elements sodium and potassium on the earth's surface. By the weathering 

 of silicate rocks, sodium carbonate is formed, which dissolves readily in 

 rain water and trickles down into the earth. The potassium, on the other 

 hand, remains with the other bases combined with silica and aluminium as 

 an insoluble double salt. The latter remains near the earth's surface, 

 while the sodium is flushed out by springs, brooks, and rivers, and carried 

 on to the ocean. This accounts for the fact that potassium salts pre- 

 dominate on the surface of the earth, while the ocean is rich in sodium 

 salts, especially the chloride. 



Now why should an increased amount of potassium salts create a demand 

 for a greater supply of sodium chloride? Bunge suggests the following 

 ingenious explanation: If a potassium salt, for example the carbonate, 

 comes in contact with sodium chloride, a partial decomposition takes 

 place, a little potassium chloride, and sodium carbonate being formed. 

 Now, as regards the inorganic salts of the blood-serum, sodium chloride 

 ranks foremost. It is found there to a considerable extent, and, as far as 

 we know, the amount is kept fairly constant. Now, on bringing the serum 

 in contact with the abundance of absorbed potassium salts obtained from 

 the vegetable nourishment, this double decomposition between the sodium 

 salt in the blood and the absorbed potassium salt will take place to some 

 extent. In this way potassium chloride is formed, and a part of the sodium 

 of the blood combines with the acid which was previously united with the 

 potassium. By this process the composition of the blood is changed. 



1 Z. Biol. 9, 104 (1873); 10, 111, 295 and 323 (1874); Lehrbuch der Physiologie der 

 Menschen, Bd. ii, p. 103 (1901). 



