782 COMPOSITION OF MILK. [BOOK n. 



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

 but for the most part exceedingly small (in man from 2/j, to 5yu). 

 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 casein ; but, though 

 undoubtedly even when the fat is removed from the milk each 

 globule remains surrounded by a layer of milk plasma, if we may 

 so call it, rich in casein, there are no adequate reasons for thinking 

 that the casein actually forms a membrane. 



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 pre- 

 cipitated 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 kreatiuin have been noted 

 by some observers, the extractives of milk, beyond the lecithin and 

 cholesteriii 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 207, 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 considerable quantity of phosphorus in the 

 proteids and in the nucleiu, 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 shewn 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. 



