446 PHYSIOLOGICAL CHEMISTRY. 



animals known as mammalia. The milk of different animals differs 

 somewhat in composition, but it always contains all the constituents 

 necessary for a normal development of the various tissues, liquids, 

 organs, etc., of the young mammal, which generally feeds exclusively 

 upon milk for a shorter or longer period of its early life. 



Milk is an opaque, aqueous solution of casein, albumin, lactose, and 

 inorganic salts, holding in suspension small globules of fat, invested, 

 most likely, with coatings of casein or with some other albuminous 

 envelope. The reaction of woman's milk and that of the herbivora 

 is normally alkaline, but that of carnivora is acid. Its specific 

 gravity ranges from 1.029 to 1.033, but may in extreme cases vary 

 between 1.018 and 1.045. 



The average composition of various kinds of milk is given below, 

 but it must be remembered that milk not only differs in certain species, 

 but also in the same animal at different times ; for instance, the 

 quality and quantity of food taken, as also various physiological 

 changes, have decided influence upon the milk secreted. 



Human milk. Cow's milk. 



The inorganic salts consist chiefly of calcium or sodium phosphate 

 and sodium and potassium chloride, but contain also some magnesium 

 and iron. The proteids consist mainly of casein with some albumin, 

 the proportion being about as 6 to 1 . 



Besides the constituents mentioned in the above analyses, milk also 

 contains a very small quantity of extractives, among which are found 



1 Including cane-sugar added by the manufacturer. 



