SECRETION OF MILK 701 



opment of the young body. We therefore find in milk proteid, fat, carbo- 

 hydrate and mineral substances. 



Among the proteids of milk we find: in small quantities lact-globulin, 

 which is probably identical with serum globulin; lact-albumin,, which is dis- 

 tinguished by certain properties from serum albumin; and,, most important 

 of all, casein. Cow's milk contains on the average about 3 per cent of casein 

 and 0.5 per cent of other proteids. 



Casein of cow's milk is a imcleo-albumin (cf. page 75) which is charac- 

 terized chiefly by the fact that it coagulates under the influence of rennin (cf. 

 page 250). In the dried state casein is a fine white powder. It is insoluble in 

 water and very difficultly soluble in solutions of the common neutral salts, but 

 in water to which a very slight trace of alkali has been added it is very readily 

 soluble. It is soluble also in the presence of calcium carbonate from which the 

 casein, acting as an acid, displaces the carbon dioxide. Its solutions do 'not 

 coagulate on boiling and are not precipitated by magnesium sulphate, metallic 

 salts, or mineral acids in excess. 



The casein of woman's milk is distinguished from that of cow's milk chiefly 

 in the form of the clot. While the clot of cow's milk is composed of dense, 

 compact masses, in the coagulation of woman's milk a very loose and finely 

 flocculent precipitate is formed. This difference, as will be readily understood, 

 is a matter of great importance for digestion in the stomach of the infant. 

 Besides, the casein of woman's milk does not always coagulate under the influ- 

 ence of rennin, and in its digestion, according to Kobrak, pseudonuclein is 

 formed in very much smaller quantities than in digestion of cow's milk casein. 



Various other circumstances point to the conclusion that the casein of 

 woman's milk is a compound of a nucleo-albumin similar to the casein of cow's 

 milk with a basic proteid body, probably a histon or a protamin. In fact it is 

 possible to prepare from the casein of woman's milk a body which in its coagu- 

 lation forms a compact cake not unlike that of cow's milk casein (Kobrak). 



Finally, there has been found in woman's milk a proteid substance, very 

 rich in sulphur and relatively poor in carbon, called opalisin (Wroblewsky), 

 which occurs in cow's milk only in very slight quantities. 



The fat of milk is present in the form of small droplets. For a long time 

 it was believed with Ascherson that these droplets were surrounded by a 

 proteid membrane (haptogen membrane). Later researches seemed to have 

 shown that this is not true, but that the fusion of milk droplets is prevented 

 only by the surface tension of the constituents of different specific gravity 

 present in milk, or by a layer of casein or proteid solution held about the 

 droplets by molecular attraction. But more recently the former view has been 

 taken up again, and it is now stated very definitely by Voltz that the milk 

 droplets possess envelopes of solid substance, which are probably true mem- 

 branes. These envelopes contain both nitrogenous and nonnitrogenous com- 

 pounds,, as well as inorganic substances of which calcium is the chief. Their 

 chemical composition varies greatly. 



In woman's milk the fat droplets are larger and their number smaller than 

 in cow's milk. 



Butter, which is the fat of milk, consists mainly of palmitin and olein. 

 Besides we find: stearic acid, myristinic acid, small quantities of butyric acid, 

 caproic acid, etc., in the form of triglj'cerides. The melting point of the fat 



