78 ALIMENTATION. 



margarine, butyrine, and one or two other unimportant va- 

 rieties) ; sugar of milk ; coloring matter, and finally, a great 

 variety of inorganic salts, including a combination of iron. 



Cows' milk, which is the variety most commonly used, 

 has, according to Payen, the following composition : l 



Composition of Cows' Milk. 



Water 86-40 



Nitrogenized substances (caseine, albumen, lacto-proteine, and matters 



soluble in alcohol) 4*30 



Lactose (sugar of milk or lactine) 5'20 



Butter (or fatty matters) 3'70 



A trace of coloring and aromatic matters. 



( Phosphate of lime ) 



Salts, slightly soluble < " " magnesia ( 0-25 



( " " iron } 



C Chloride of sodium } 



Soluble salts < " " potassium V 0-15 



( Phosphate and lactate of soda ) 



100-00 



The quality of milk is usually considered with reference 

 to its reaction, specific gravity, and its proportion of fatty 

 matter. When perfectly fresh it is neutral or sometimes 

 slightly alkaline. In a short time it becomes faintly acid, and 

 its acidity is increased by the transformation of a portion of its 

 sugar into lactic acid, until this becomes sufficient to coag- 

 ulate the caseine. It is then said to be soured, and separates 

 into the curd and whey. In this state it is not unwholesome, 

 1 Op. dt, p. 139. 



It is hardly necessary to describe fully the other varieties of milk which are 

 used as food. The milk of the goat, ass, mare, reindeer, and sheep are some- 

 times used, and present certain differences from cows' milk. In general, the 

 odorous principles are distinctive. Goats' milk contains much more butter and a 

 little more caseine and sugar than cows' milk. Ewes' milk contains a large excess 

 of caseine and butter, but is rather deficient in sugar. Asses' milk contains a rel- 

 atively small proportion of caseine and butter, and a large proportion of sugar. 

 Mares' milk contains a very large proportion of sugar (8'75 per hundred), a small 

 proportion of caseine, and very little butter. Human milk contains less caseine 

 and more sugar and butter than cows' milk. This will be more fully considered 

 in the section devoted to secretion. 



