CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 781 



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, 207, 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 coagulation with 

 heat, or by precipitation with potassium ferrocyanide, &c. In the 

 process of curdling the casein, as stated in 207, 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 peptone or albumose. 

 This or a similar peptone-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 in the superficial layers exposed to the air changes 

 take place by which a film or skin, derived chiefly from the 

 albumin but partly from the casein, appears on the surface ; if this 

 be removed a fresh portion undergoes the same change. The 

 peculiar body nuclein which as we have seen, 29, is a complex 

 nitrogenous body differing in composition from proteids, is also 

 present in milk in small quantities, and according to some 

 observers is simply suspended, not really in solution, or is in some 

 way peculiarly associated with the casein. 



</_ 2. Fats. These are, in the main, palmitin, stearin, and olein ; 

 but other fats, supplied by butyric and other fatty acids in combi- 

 nation with glycerine, accompany the above in small quantities. 

 In this respect the fat of milk resembles that of adipose tissue. 

 Lecithin and cholesterin are also present in very small quantity, 

 as well as a yellow colouring matter. The fat present in milk 

 differs in different animals as to the relative proportion of olein, 

 palmitin and stearin, and as to the kinds and relative amount 

 of the other scantier fats. 



The mixture of these fats, fluid at ordinary temperatures, is 



