430 MILK. [BOOK 11. 



*2. Fats. These are, in human milk, palmitin, stearin, and 

 olein. But other fats are also present in small quantities ; and the 

 composition of the fats of milk differ in different animals. 



3. Milk-sugar, the conversion of which into lactic acid gives 

 rise to many of the features of milk. 



4. Extractives, including, according to some observers, urea, 

 and salts. The last consists chiefly of potassium phosphate, with 

 calcium phosphate, potassium chloride, small quantities of mag- 

 nesium phosphate, and traces of iron. 



The following is the composition of 1000 parts of 

 Human Milk. Cow's Milk. 



Milk is an emulsion, the fats existing in the form of globules of 

 various but minute size, each protected by a thin envelope of 

 casein or albumin. It is this condition of the fat which gives to 

 milk its peculiar white colour. The colostrum, or secretion of the 

 mammary gland at the beginning of lactation, differs from milk in 

 being very deficient in casein and proportionately rich in albumin. 

 It is said that the milk at the end of a long lactation again 

 becomes poor in casein and rich in albumin. Milk on standing 

 turns sour and curdles. This is generally due to the milk-sugar 

 becoming converted by a fermentative process into lactic acid, which 

 in turn precipitates the casein. Curdling may however as we have 

 already seen (p. 246) take place by the action of rennet ferment 

 quite independently of the production of any acid. 



Milk, like the other secretions which we have studied, is the 

 result of the activity of certain protoplasmic secreting cells forming 

 the epithelium of the mammary gland. As far as the fat of milk 

 is concerned, the processes taking place in the gland are very 

 instructive, since the fat can be seen to be gathered in the 

 epithelium-cell, in the same way as in a fat-cell of the adipose 

 tissue, and to be discharged into the channels of the gland, either 

 by breaking away from the cell, or by a contractile extrusion very 

 similar to that which takes place when an amoaba ejects its digested 

 food. All the evidence we possess goes to prove that the fat 

 is formed in the cell through a metabolism of the protoplasm. 

 The microscopic history is thoroughly supported by other facts. 

 Thus the quantity of fat present in milk is largely and directly 

 increased by proteid, but not increased, on the contrary diminished, 

 by fatty food. This is quite intelligible when we know, as will be 



