818 COMPOSITION OF MILK. [BOOK n. 



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 

 caseinogen, 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 fermen- 

 tation into lactic acid, through the agency of an organized ferment ; 

 the milk thus becomes sour, and the caseinogen 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 already mentioned, are insignificant. The 

 salts are of more importance ; these are chiefly calcium phosphate, 

 of whose function in the process of curdling we spoke in 207, and 

 potassium and sodium chlorides, with a small quantity of magnesium 

 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 phos- 

 phates, milk contains a further considerable quantity of phosphorus 

 as well as some sulphur in the caseinogen (nucleo-albumin). 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. 



Average Composition of Milk in Different Animals. 



Woman. Cow. Mare. Bitch. 



Casein &c. 2 4 2*5 10 



Fats 2-75 4 2 10 



Sugar 5 4*4 5 3*5 



Salts -25 -6 -5 -5 



Total Solids 10 13 10 24 



Water 90 87 90 76 



