436 THE HUMAN BODY 



which when moistened with water forms a tenacious mass, and 

 this it is to which wheaten bread owes its superiority. When the 

 dough is made, yeast is added to it, and produces a fermentation 

 by which, among other things, carbon dioxid gas is produced. 

 This gas, imprisoned in the tenacious dough, and expanded during 

 baking, forms cavities in it and causes it to "rise" and make 

 "light bread," which is not only more pleasant to eat but more 

 digestible than heavy bread. Other cereals may contain a larger 

 percentage of starch, but none have. so much gluten as wheat; 

 when bread is made from them the carbon dioxid gas escapes so 

 readily from the less tenacious dough that it does not expand the 

 mass properly. Corn and rice are valuable chiefly for their high 

 carbohydrate content; beans and peas, on the other hand, have 

 a high per cent of protein. Potatoes contain less actual nutri- 

 ment for their weight than do any of the other important foods. 

 Their cheapness and digestibility have combined to give them a 

 place in the average dietary out of all proportion to their real 

 value. Other fresh vegetables, as carrots, turnips, and cabbages, 

 are valuable mainly for the salts they contain; their weight is 

 mainly due to water, and they contain but little starch, proteins, 

 or fats. Fruits, like most fresh vegetables, are mainly valuable 

 for their saline constituents, the other food stuffs in them being 

 only, present in small proportion. The cellulose which they con- 

 tain makes up the major portion of the roughage of the diet, and 

 is valuable on that account. 



The Cooking of Vegetables. This is of more importance even 

 than the cooking of flesh, since in most the main alimentary 

 principle is starch, and raw starch is difficult of digestion. In 

 plants starch is stored up within the walls of the plant-cells, which 

 are of cellulose and therefore indigestible. When vegetables are 

 cooked the contents of the cells swell, the cellulose walls are rup- 

 tured and the starch is set free to be acted upon by the digestive 

 mechanism of the Body. 



Composition of Foods. The following table gives the per- 

 centage composition of some of the common foods. 



