612 THE NATURE OF MILK. [Book ii. 



' colostrum corpuscles ' characteristic of the first milk, of which 

 we shall speak presently. 



At the end of lactation an absorption of some of the alveoli 

 takes place ; and in old age still further absorption goes on 

 with great diminution of the lumina. 



§ 406. In the lymphatic spaces of the connective tissue 

 which joins together the lobules of various sizes, surrounds the 

 lobules and runs in between the projecting blind ends of the 

 alveoli within the lobules leucocytes are numerous, and some 

 of these may make their way through the basement membrane 

 and between the secreting cells into the cavities of the alveoli 

 and so appear in the milk. 



§ 407. The nature of milk. Human milk has a specific 

 gravity of from 1-028 to 1-034, and when quite fresh possesses 

 a slightly alkaline reaction. It speedily becomes acid ; and 

 cow's milk, even when quite fresh, is sometimes slightly acid, 

 the change of reaction taking place during the stagnation of 

 the milk in the mammary ducts. 



The constituents of milk are: 



1. Proteids, viz. casein, and an albumin, agreeing in its 

 general features with ordinary serum-albumin, but which, since 

 it is said to differ somewhat in its solubilities and rotatory 

 power from serum-albumin, has been called lactalbumin. The 

 casein, as we have seen, § 185, undergoes through the action of 

 rennin a change whereby insoluble casein (tyrein) makes its 

 appearance and the milk is curdled. Casein may however be 

 precipitated in an unchanged form by saturating milk with 

 neutral salts, or by the careful addition of acetic acid to diluted 

 milk, or by first adding to the diluted milk a slight quantity 

 of acetic acid and then passing through it a stream of carbonic 

 acid. In the filtrate the presence of the lactalbumin, which 

 occurs in small and variable quantities, may be shewn by coagu- 

 lation with heat, or by precipitation with potassium ferro- 

 cyanide, &c. In the process of curdling the casein, as stated 

 in § 185, appears to be not simply changed into tyrein but to 

 be split up into tyrein and into another proteid, which unlike 

 the lactalbumin is not coagulated by heat and which appears to 

 be allied to albumose. This or a similar albumose-like body has 

 also been found in small quantities even in milk which has not 

 curdled; it has been called lactoprotein. The lactalbumin, 

 though coagulated by heat when isolated, is not so coagulated 

 as it exists in the natural milk, the alkalinity of the milk, which 

 is increased by boiling, preventing this. Similarly casein, 

 though coagulated by heat when simply suspended in water 

 after being precipitated, is not coagulated by heat when it exists 

 in a natural condition in milk; in these respects casein behaves 

 like alkali-albumin, which it resembles in other features also. 

 Hence milk when boiled does not coagulate as a whole, though 



