Chap, it.] METABOLIC PROCESSES OF THE BODY. 613 



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 re- 

 moved 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 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 

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

 but for the most part exceedingly small (in man from 2/x to 

 5/x). Milk is in fact a typical emulsion, 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 fer- 

 mentation into lactic acid, through the agency of an organized 

 ferment; the milk thus becomes sour, and the casein is precipi- 

 tated 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 already mentioned, are insignificant. 

 The salts are of more importance; these are chiefly calcic 

 phosphate, of whose function in the process of curdling we 

 spoke in § 185, 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 sulpho-cyanide have been observed. Besides the phosphorus 

 in the actual form of phosphates, milk contains a further con- 

 siderable quantity of phosphorus in the proteids 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 re- 

 semble those of the entire body. 



The composition of milk in the same animal varies widebj 



