CONDITIONS WHICH MODIFY THE LACTEAL SECRETION 293 



salts ; and milk furnishes all the inorganic matter necessary for the nutri- 

 tion of the infant, even containing a small quantity of iron. 



The lactose, or sugar of milk, the casein and the fatty particles are 

 all produced in the gland. The peculiar 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 bloodvessels of a living animal, are not 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. 



Casein is produced in the mammary glands, probably by a peculiar 

 transformation of the proteid constituents of the blood. The fatty 

 globules are likewise produced in the substance of the gland, and the 

 peculiar kind of fat that exists in this secretion is not found in the blood. 

 The mechanism of the production of fat in the mammary glands is some- 

 what obscure. During full activity of the lacteal secretion, however, 

 small fatty globules appear in the cells lining the alveoli, particularly 

 that portion of the cells next the lumen, and these finally are discharged 

 into the ducts. 



As regards the mechanism of the formation of the peculiar and 

 characteristic constituents of milk, the mammary glands are to be 

 classed among the organs of secretion and not with those of elimina- 

 tion, or excretion ; for none of these matters preexist in the blood and 

 they all appear first in the substance of the glands. 



During the period of secretion, the glands receive a larger supply of 

 blood than at other times. Pregnancy favors the development of the 

 secreting portions of the glands but does not induce secretion. On the 

 contrary, when pregnancy occurs during lactation, it diminishes and 

 modifies, and it may arrest the secretion of milk. The secreting action 

 of the mammary glands is nearly continuous. When the secretion of 

 milk has become fully established, while there may be certain times 

 when it is formed in greater quantity than at others, there is no actual 

 intermission in its production. 



General Conditions which modify the Lacteal Secretion. Very little 

 is known concerning the physiological conditions which modify the 

 secretion of milk. When lactation is fully established, the quantity and 

 quality of the milk secreted become adapted to the requirements of the 

 child at different periods of its existence. In studying the composition 

 of the milk, therefore, it will be found to vary considerably in different 

 stages of lactation. It is evident that as the development of the child 

 advances, a constant increase of nourishment is demanded ; and as a 

 rule the mother is capable of supplying all the nutritive requirements of 

 the infant for eight to twenty months. 



