CONSTITUENTS OF FOOD. 99 



seen, the advantage in muscular strength would be all the 

 other way. Also the work of digesting hay, to take it as an 

 appropriate instance, necessitates, according to Zuntz and 

 Lehmann, the expenditure of 11.2 per cent, of the energy pro- 

 duced by the food ; and oats, only 2.8 per cent. Furthermore, 

 the large size of the stomach and intestines would greatly 

 interfere with the animal's powers of breathing during severe 

 labour. 



The fact that food rich in fibre necessitates for its mastication 

 the secretion of much more saliva, and consequently the drink- 

 ing of more water than does corn, accounts to a great extent 

 for the marked effect which its consumption has in increasing 

 the weight of the contents of the stomach and intestines. 



Among mineral matters, phosphoric acid, potash, soda 

 (mostly as common salt), lime, and iron play the most impor- 

 tant part in nutrition. Phosphoric acid, potash, and iron are 

 plentiful in hay and corn, in both of which the percentage of 

 soda is less than the requirements of the system. In the entire 

 body of the higher animals, the quantity of potash is about 

 equal to that of soda. Blood contains much more soda 

 than potash ; and bloodless meat, more potash than soda. 

 Lehmann tells us that the mixture of blood and flesh which 

 carnivorous animals consume, contains about three times more 

 potash than soda. On this diet, these animals show no 

 craving for common salt ; neither do human beings who live 

 almost exclusively on meat. But persons who live principally 

 on potatoes, which contain a large percentage of potash and 

 little or no soda, eat salt with great relish and find that 

 it is necessary to their health. Horses, whose ordinary 

 mixture of hay and corn contains a comparatively small 

 amount of soda, also crave for salt, and salt added to their 

 food has an obviously good effect on their condition. In the 

 food of plant-eaters (horses, for example), there appears to be 

 an excess of potash, which tends to irritate the mucous mem- 



7* 



