FOOD AND DIGESTION 335 



procedures is to diminish the amount of water, and thus to 

 increase the solids. 



3. Eggs. The egg of the domestic fowl need alone be 

 considered. The composition of the white and of the yolk 

 naturally differs considerably. The white of egg is nothing 

 more than a solution of proteins. 



In the yolk there is a very large amount of lecithin (p. 397) 

 along with ordinary fats, and a large amount of a phospho- 

 protein ; and the great value of eggs is thus that they 

 contain both proteins, ordinary fats, and the special phosphorus 

 containing protein and fat. The mixed contents of the egg 

 contains a little more than 10 per cent, each of proteins and 

 of fats. 



Speaking generally, we may say that the animal food-stuffs 

 are rich in proteins and fats, but are poor in carbohydrates. 



Vegetable Food-stuffs. The peculiarity of special importance 

 in vegetables is the existence of a capsule to the cells, com- 

 posed of cellulose a substance allied in its composition to 

 starch or of lignin or allied substances. Cellulose is to 

 a large extent dissolved and decomposed in the alimentary 

 canal of herbivora, while it is practically unacted upon in 

 man and in the carnivora. Lignin or woody matter resists 

 digestive changes even in the herbivora, and its only value is to 

 increase the mass of the faeces, and thus to stimulate intestinal 

 action and to act as a natural purgative. 



The chief vegetable foods of the herbivora are grass, hay, 

 oats, maize, and the leguminous plants, such as peas and 

 beans. 



These vary considerably in composition according to their 

 character, the ground in which they have been grown, and 

 the season of the year at which they are used. Further, the 

 methods of analysis do not give absolutely definite results. 

 The proteins are generally estimated by determining the 

 nitrogen and multiplying by 6'25. But other nitrogen- 

 containing substances besides proteins occur in plants, amido- 

 acids such as asparagin, which, while they are burned in 

 the body and may yield energy like the fats and carbo- 

 hydrates, do not seem capable of being used like the proteins 

 in building up the living tissues. The amount of these sub- 



