MILK. 81 



all the inorganic matter necessary for the nutrition of the 

 infant, containing, even, a small quantity of iron. Pre- 

 cisely how the secreting vesicles separate the proper quan- 

 tity of these principles from, the circulating fluid, we are 

 unable, in the present state of our knowledge, to determine. 

 It is unsatisfactory enough to say that the membranes of 

 the vesicles have an elective action, but this expresses the 

 extent of our information on the subject. 



The lactose, or sugar of milk, the caseine, and the fatty 

 particles, are all produced, de novo, in the gland. The pecu- 

 liar kind of sugar here found does not exist anywhere else in 

 the organism. Even when the secretion of milk is most 

 active, different varieties of sugar, such as glucose or cane- 

 sugar, injected into the blood-vessels of a living animal, are 

 never eliminated by the mammary glands, as they are by the 

 kidneys ; and their presence in the blood does not influence 

 the quantity of lactose found in the milk. All that can be 

 said with regard to the formation of sugar of milk is, that it 

 is produced in the mammary glands. The mechanism of its 

 formation is not understood. 



Caseine is produced in the mammary glands, probably 

 by a catalytic transformation of the albuminoid constituents 

 of the blood. This principle does not exist in the blood, 

 though its presence here has been indicated by some observ- 

 ers. The substance in the blood that has been mistaken 

 for caseine is undoubtedly albumen, which will not respond 

 to some of the tests on account of the alkalinity of the fluid 

 in which it is contained. It is well known that the caseine 

 of milk is precipitated by an excess of sulphate of magnesia ; 

 but the so-called caseine of the blood is not affected by this 

 salt, and passes through it like albumen. 1 



The fatty particles of the milk are likewise produced in 

 the substance of the gland, and the peculiar kind of fat 

 which exists in this secretion is not found in the blood. 

 The mechanism of the production of fat in the mammary 



1 LONGET, Traite de physiologic, Paris, 1869, tome ii., p. 283. 

 6 



