CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 817 



The constituents of milk are : 



1. Proteids, viz. casein, or caseinogen, 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 

 caseinogen, as we have seen, 207, undergoes through the action 

 of rennin a change whereby insoluble casein makes its appearance 

 and the milk is curdled. Caseinogen 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 precipi- 

 tation with potassium ferrocyanide, &c. In the process of curdling 

 the caseinogen, as stated in 207, appears to be not simply changed 

 into casein but to be split up into casein and into another proteid, 

 which unlike the lactalbumin is not coagulated by heat and which 

 appears to be allied to peptone or albumose. 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 caseinogen, 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. 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 caseinogen, appears on the surface ; if this be 

 removed a fresh portion undergoes the same change. Nuclein is 

 also said to be present in small quantities, but this may be simply 

 derived from the caseinogen, which appears ( 207) to be of the 

 nature of nucleo-albumin. 



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 glycerin, 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 

 present in natural milk in the form of globules of various sizes but 

 for the most part exceedingly small (in man from 2/u, to S/JL). Milk 

 is in fact a typical emulsion, and it is the presence of the casein 

 in the milk which brings about the emulsion. Some observers 

 maintain that each globule of fat is surrounded by an envelope or 

 membrane of solid undissolved caseinogen; but, though undoubtedly 



