THE CONSTITUENTS OF MILK 15 



formed in the body Ijy a breaking down of the galac- 

 tins of the food. Others regard lactose as a product 

 of certain proteids (glycoproteids). Milk fat is de- 

 rived partly from the fat in the food, partly from the 

 fat tissues of the animal; but these fats undergo a 

 material transformation in the tissues of the udder, 

 so that certain easily recognizable fats, even when taken 

 up in quantity with the food, are either not visible at all 

 in the milk or appear in very small quantity or are 

 merely transitory. Doubtless, milk fat — just as fat tis- 

 sue — may also be derived from the car1)ohydrates of the 

 food. Among the other ingredients of milk, citric 

 acid does not originate in the food, but results from 

 metabolism. 



Milk secietion, to a great degree, bears the impres- 

 sion of specific action of the cells which, however, may 

 be influenced by external circumstances, but usually only 

 temporarily. This is naturally of great importance for 

 the young animal, which is nourished entirely or chiefly 

 on its mother's milk, as frequent and sudden changes 

 in the composition of the milk would be harmful, if not 

 dangerous. 



Milk secretion, it appears, may be checked through 

 the influence of the central nervous system, but is regu- 

 lated through the sympathetic nerve centres. In this 

 connection, observations agree that certain irritant 

 drugs (as pilocarpine) and other influences that decid- 

 edly affect the secretory function of other glands, influ- 

 ence milk secretion very little if at all; furthermore, 

 section of the nerve trunks of the udder does not disturb 

 secretion. 



II. THE CONSTITUENTS OF MILK 



As already mentioned, milk of different mammals 

 consists essentially of the same materials, namely: 

 water, protein, sugar, fats and inorganic matter; but 



