322 THE HUMAN BODY. 



thin, or rather a substance yielding it, is an important con- 

 stituent of the nervous tissues. 



Milk contains a proteid, caseinogen; several fats in the 

 butter; a carbohydrate, milk-sugar; much water; and salts, 

 especially potassium and calcium phosphates. Butter consists 

 mainly of the same fats as those in beef and mutton; but has 

 in it about one per cent of a special fat, butyrin. In the milk 

 it is disseminated in the form of minute globules which, for 

 the most part, float up to the top when the milk is let stand 

 and then form the cream. In this each fat-droplet is sur- 

 rounded by a pellicle of albuminous matter; by churning, 

 these pellicles are broken up and the fat-droplets then run to- 

 gether to form the butter. Caseinogen is insoluble in water; 

 in milk it is dissolved by the alkaline salts present. When 

 milk is kept, its sugar ferments and gives rise to lactic acid, 

 which neutralizes the alkali and precipitates the caseinogen 

 as curds. In cheese-making the caseinogen is acted upon by 

 a ferment (rennin) present in the extract of stomach used, 

 and converted into tyrein which is precipitated : this clotting 

 does not take place unless a calcium salt be present. Tyrein, 

 which forms the main bulk of a true cheese, is different from 

 the curd precipitated from milk by acids; cheese made from 

 the latter does not " ripen.'' Caseinogen is frequently called 

 casein, which name should be given to the tyrein formed from 

 caseinogen by ferment action. 



Vegetable Foods. Of these wheat affords the best. In 

 1000 parts it contains 135 of proteids, 568 of starch, 46 of 

 dextrin (a carbohydrate), 49 of grape-sugar, 19 of fats, and 

 32 of cellulose, the remainder being water and salts. The 

 proteid of wheat is mainly gluten, 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 dioxide 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. Other cereals may con- 

 tain a larger percentage of starch, but none have so much 

 gluten as wheat; when bread is made from them the carbon 

 dioxide gas escapes so readily from the less tenacious dough 

 that it does not expand the mass properly. Corn contains in 



