THE MAMMARY GLAND. 479 



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 con- 

 dition in milk ; in these respects casein behaves like alkali-albumin, which it 

 resembles in other features also. Hence milk when boiled does not coagu- 

 late 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. 



2. Fats. These are, in the main, palmitin, stearin, and olein ; but other 

 fats, supplied by butyric and other fatty acids in combination 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 coloring 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 // to 5 />.). Milk is in fact a typical emul- 

 sion, and it is the presence of the casein in the milk which brings about the 

 emulsion. 



On standing a great deal of the fat collects on the top of the milk in the 

 form of cream, but in this, as in the butter which is formed from it, the 

 globules are still discrete, so long at least as the butter is " fresh." By the 

 use of a centrifugal machine nearly the whole of the fat may be separated 

 from the plasma. 



3. Milk sugar or lactose. This is very apt to undergo fermentation into 

 lactic acid, through the agency of an organized ferment ; the milk thus 

 becomes sour, and the casein is precipitated in a flocculent form when the 

 acid is produced in sufficient quantity. Since the change will take place 

 even when every care is taken to exclude germs from the atmosphere having 

 access to the milk, the organized ferments must be present in the milk in the 

 ducts of the gland. 



4. Salts. Though traces of urea and kreatinin have been noted by some 

 observers, the extractives of milk, beyond the lecithin and cholesterin al- 

 ready mentioned, are insignificant. The salts are of more importance ; 

 these are chiefly calcic phosphate, of whose function in the process of curd- 

 ling we spoke in 193, and potassic and sodic chlorides, with a small 

 quantity of magnesic phosphate. Sulphates appear to be absent. A small 

 quantity of an iron salt is present, and traces of sulphocyanide have 

 been observed. Besides the phosphorus in the actual form of phosphates, 

 milk contains a further considerable quantity of phosphorus in the pro- 

 teids and in the nuclein, as well as some sulphur in the former. The 

 inorganic constituents of milk may, broadly speaking, be said to differ 

 distinctly from those of blood, and to much more nearly resemble those 

 of the entire body. 



The composition of milk in the same animal varies widely from time to 

 time, and besides undergoes marked changes during the period of lactation. 

 The relative general composition of human milk and that of the cow, the 

 mare, and the bitch may perhaps be shown by the following table ; but it is 

 difficult to draw an average, since the individual analyses given differ so 

 much ; the figures given for casein and fat in the milk of the bitch may be 

 unusually high. 



