328 HANDBOOK OF PHYSIOLOGY. 



that except for difference of flavor, etc., the average composition is 

 nearly the same in every case. 



b. Milk.* Is intended as the entire food of young animals, and as 

 such contains, when pure, all the elements of a typical diet. (1) Albu- 

 minous substances in the form of caseinogen, and serum or lact-albumin. 

 (2) Fats in the cream. (3) Carbo-hydrates in the form of lactose or milk 

 sugar. (4) Salts, chiefly calcium phosphate; and (5) Water. From it we 

 obtain (a) cheese, which is the clotted caseinogen or casein precipitated 

 with more or less of fat according as the cheese is made of skim milk 

 (skim cheese), of fresh milk with its cream (Cheddar and Cheshire), or of 

 fresh milk plus cream (Stilton and double Gloucester). The precipi- 

 tated casein is allowed to ripen, by which process some of the al- 

 bumin is further split up, with formation of fat. (/5) Cream, consists of 

 the fatty globules encased in caseinogen and serum-albumin, and which 

 being of low specific gravity float to the surface. (Y} Butter, or the 

 fatty matter deprived of its proteid envelope by the process of churning. 

 (8) Buttermilk, or the fluid obtained from cre,am after butter has been 

 formed; very rich therefore in nitrogen, (s) Whey, OY the fluid which 

 remains after the precipitation of casein; it contains sugar, salt, and a 

 small quantity of albumin. 



TABLE OF COMPOSITION OF MILK, BUTTER-MILK, CREAM, AND CHEESE. (LETHEBY 



AND PAYEN.) 



Nitrogenous matters. Fats. Lactose. Salts. Water. 



Milk (Cow) .... 4.1 3.9 5.2 .8 86 



Buttermilk .... 4.1 .7 6.4 .8 88 



Cream 2.7 26.7 2.8 1.8 66 



Cheese. Skim ... 44.8 6.3 4.9 44 



Cheese. Cheddar ... 28.4 81.1 4.5 36 



Non-nitrogenous 

 matter and loss. 



Cheese. Neufchatel ( Fresh) . 8. 40.71 36.58 .51 36.58 



c. Eggs. The yolk and albumen of eggs are in the same relation as 

 food for the embryos of oviparous animals that milk is to the young of 

 mammalia, and afford another example of the natural admixture of 

 the various alimentary principles. The proteids of eggs are egg-albumin 

 and globulins, of which the vitellin of the yolk is most important; 

 nuclein in combination with iron is also found. In addition to the 

 three common fats there is a yellow fat, lutein (lipochome), a small 

 quantity of grape sugar; lecithin, and cholesterin and inorganic salts, 

 chiefly potassium chloride and phosphates. 



TABLE OF THE PERCENTAGE COMPOSITION OF FOWLS' EGGS. 



Nitrogenous substances. Fats. Salts. Water. 



White 20.4 _ 1.6 78 



Yolk .... 16. 30^7 1.3 52 



* The details of the composition of milk have been discussed in the Chapter on 

 Secretion. 



