FOODS. 149 



The vegetable foods, as a class, vary considerably in nutritive 

 value and digestibility, the latter depending on the amount of cellu- 

 lose they contain. A section of a vegetable shows not only the pres- 

 ence of an external cellulose envelope, but also an inner framework 

 which penetrates its substance in all directions. The nutritive 

 principles are contained in small cavities, the walls of which are 

 formed by the framework. Nearly all vegetables require cooking 

 before being eaten. When subjected to heat and moisture, not only 

 is the texture of the vegetable softened and disintegrated, but the 

 starch grains are hydrated and partially prepared for conversion 

 into dextrin and sugar. At the same time various savory substances 

 are set free, which make the food more palatable. 



Beans and peas contain large quantities of a proteid, legumin, 

 and starch, and hence are especially valuable as nutritive foods. 

 The presence of the cellulose envelope, especially in ripe beans and 

 peas, combined with rather a dense texture, renders them somewhat 

 difficult of digestion. Potatoes, though largely employed as food, 

 are extremely poor in proteids, 2 per cent., and carbohydrates, 20 per 

 cent. ~When sufficiently cdbked they are easily digested, owing to 

 the small amount of cellulose they contain. 



Green vegetables, e. g., lettuce, spinach, tomatoes, asparagus, 

 onions, etc., though containing food principles in small amounts, 

 are, nevertheless, valuable adjuncts to the dietary, for the reason 

 that they contain inorganic as well as organic salts, which appear 

 to be .necessary to the maintenance of the normal nutrition. The 

 want of green vegetables has been supposed to be the cause of 

 scurvy. 



Ripe fruits, grapes, cherries, apples, pears, peaches, strawberries, 

 lemons, oranges, etc., though consumed largely, possess but little 

 nutritive value. They consist largely of water, 75 to 85 per cent., 

 proteids a trace, sugar from 5 to 13 per cent., organic acids (citric, 

 malic, tartaric), pectose, and various inorganic salts. 



Relative Value of Animal and Vegetable Foods. Though 

 both animal and vegetable foods contain the different classes of food 

 principles, it is not a matter of entire indifference as to which are 

 consumed. It has been found by experiment that animal proteids 

 are more easily and completely digested and absorbed than vegetable 

 proteids ; that cellulose is not only highly indigestible, but by its pres- 

 ence in large quantities retards the digestive process and impairs the 

 activity of the entire digestive mechanism, though in moderate quan- 

 tity it undoubtedly aids digestion indirectly by mechanically pro- 

 moting peristalsis. The following table shows the relative diges- 

 tibility of the two classes of foods: 



