WATER AND SODIUM CHLORIDE 35 



impelled to seek an additional supply. This fact is the more remark- 

 able because all of our ordinary articles of food contain abundance of 

 sodium chloride, yet however much of other diet we may eat we still 

 experience salt-hunger, a hunger which under certain conditions, may 

 become positively distressing. 



In this connection it is noteworthy that a very close parallelism 

 exists between the nature of the diet of different animals and peoples 

 and their requirement of salt; a parallelism which was first pointed out 

 and interpreted by the physiological chemist von Bunge. 



Very many of the animals whose diet is purely vegetarian experience 

 a desire for salt. Carnivorous animals, on the contrary, such as the 

 dog or cat, not only do not desire salt, but actually exhibit an aversion 

 for salted food. This is very well illustrated by well-known habits of 

 many of the wild animals. It is a fact commented upon in nearly every 

 book of traveller's and hunter's tales, that the hoofed animals, the deer 

 and so forth, of which the dietary is exclusively vegetable, deliberately 

 seek for salt, in salt-pools and efflorescences, where they lick the salt, 

 and will travel very long distances to do so. As all readers of travel 

 and adventure know, it is at salt licks that hunters watch for such 

 game. On the other hand, salt has never any attraction for the wild 

 beasts of prey. 



This difference of behavior becomes all the more striking when we 

 reflect upon the fact that, weight for weight, a herbivorous animal 

 takes in with its ordinary food just about the same quantity of sodium 

 and chlorine per day as a carnivorous animal. Each receives the same 

 allowance of salt. Yet the herbivora experience a longing for more 

 salt and the carnivora do not. 



The reason is obviously to be sought in some other difference between 

 their contents of sodium chloride. Now one very striking difference 

 is found between the mineral contents of vegetable and animal food. 

 Vegetables nearly all contain a superabundance of potassium salts. 

 Animal flesh, on the contrary, contains sodium and potassium in nearly 

 equal proportions, so that although a herbivorous animal obtains just 

 as much sodium chloride per day as a carnivorous animal, .yet it 

 obtains in many cases no less than six times as much potassium as a 

 carnivorous animal does. It is in fact a general rule, to which there are 

 but few exceptions, that in plant-tissues potassium predominates very 

 greatly over sodium, while in animal tissues these mineral bases are 

 present in approximately equal proportion. 



Von Bunge sought to trace the origin of the craving which herbiv- 

 orous animals experience for salt to the excess of potassium in the diet. 

 Suppose that a salt of potassium, say potassium citrate, gains entrance 

 into the blood by having been ingested with the food. On arriving in 

 the blood-stream, the potassium citrate meets with an excess of sodium 

 chloride, for in the blood-plasma, or fluid part of the blood, sodium 

 predominates very greatly over potassium. Of course a certain degree 

 of interchange of ions will take place. A proportion of the potassium 



