234 MILK ! ITS ORIGIN AND COMPOSITION 



undergo disintegration and partial solution in the more 

 liquid portion of the secretion. From the above it will 

 be gathered that milk is not merely a secretion like 

 saliva or gastric juice exuded from the secreting cells, 

 but consists of a certain amount of a liquid secretion, 

 together with solid portions of the secreting cells them- 

 selves, which have undergone transformation and a kind 

 of fatty degeneration and then been cast off into the 

 previously accumulated liquid in the alveoli. The 

 materials from which the secreting cells manufacture 

 milk are derived through the lymph from the blood 

 stream which circulates in an extensive network of 

 capillary blood vessels surrounding the alveoli. Few 

 if any materials are passed on into the milk from the 

 blood stream direct without transformation, but white 

 blood corpuscles or leucocytes often make their way 

 between the secreting cells from the blood into the milk, 

 in which they may be easily recognized. 



2. Composition of Milk. From the foregoing account 

 of the formation of milk it may be inferred that its com- 

 position must necessarily be complex. It contains a 

 large number of organic compounds both solid and in 

 solution. When freshly drawn it possesses a so-called 

 amphoteric reaction, i.e. it is acid to blue litmus and 

 alkaline to turmeric paper, a peculiarity associated with 

 the presence in it of acid phosphates, citrates and certain 

 proteins. The chief constituents present in cows' milk 

 are water, proteins, fats, sugar and mineral materials, 

 with a very minute trace of colouring substance, the 

 following representing the general average composition 

 according to Richmond : 



Water . . . . 87.10 



Proteins .... 3.40 



