CONSTITUENTS OF THE MILK. 763 



has a larger proportion of sugar and saline constituents. It exercises 

 an aperient effect upon the new-born infant. The colostric condition 

 sometimes persists for too long a period, and then the milk is less suit- 

 able for food. 



As long ago remarked by Prout, milk presents us with a type or 

 pattern of food, for it contains, in definite and duly balanced propor- 

 tions, nitrogenous and non-nitrogenous nutrient substances, albumin- 

 oid, fatty and saccharine, fitted for both plastic and respiratory pur- 

 poses, and, besides these, suitable salts for the blood and tissues. 

 Milk, as we have seen, is composed of water which holds in solution 

 lactin or milk-sugar, casein or the albuminoid substance characteristic 

 of this secretion, certain extractive matters and salts ; whilst it con- 

 tains, in suspension, fatty with albuminoid matter. When set aside in 

 quantity, a natural analysis of milk takes place ; first, the oily matter, 

 being of light specific gravity, together with a certain amount of casein, 

 and even sugar and saline substances, rises as cream, the globules of 

 which, by agitation, as in the process of churning, combines to form 

 butter, leaving most of the casein, the sugar, and other substances, 

 extractives, and salts, in the buttermilk. After a time, some of the 

 sugar in this buttermilk undergoes a peculiar fermentation, perhaps 

 excited by the casein, and is changed into lactic acid ; this immedi- 

 ately precipitates the casein in minute flocculi, which combines to form 

 the so-called curd. The residual fluid, called the whey, now contains 

 most of the lactin or sugar of milk, with lactic acid, extractives, and 

 salts. x 



The fatty matter of the cream consists chiefly of olein, but it also 

 contains stearin, and, in particular, a peculiar fat, named butyrin, 

 which is a compound of butyric acid and glycerin, and imparts to but- 

 ter its characteristic taste and smell. It yields, when acted on by al- 

 kalies, and also when spontaneously decomposed at high temperatures, 

 besides butyric, small quantities of caproic and capric acids. The 

 casein of human milk is not so easily precipitated by acids or by ren- 

 net, as the casein of cow's milk ; in this respect, and also as regards 

 its smaller quantity of casein, human milk resembles more closely the 

 milk of the ass. The lactin or sugar of milk, which may be separated 

 by crystallization from inspissated whey, is convertible into grape- 

 sugar, by dilute mineral, or by vegetable, acids ; it is very prone to 

 enter into the lactic fermentation, and even to form butyric acid by 

 decomposition ; but it is difficult to transform it into alcohol. The 

 extractives of milk have not been well examined. The salts resemble 

 those of the blood; but they present curiously a larger relative amount 

 of the earthy phosphates of lime and magnesia, which are combined 

 with, and rendered soluble by, the casein. Chlorides of sodium and 

 potassium, and traces of phosphate of iron, are met with. Human 

 milk may be either neutral, alkaline, or acid ; but the milk of most 

 animals usually, and that of the Carnivora always, at the time of its 

 examination, is acid, from the presence of free lactic acid. 



The casein is probably formed by the secreting power of the mam- 

 mary gland cells, from the albuminoid principles of the blood ; but, 

 according to some, it is preformed in the blood itself during the period 



