330 SECRETION. 



gitudinally and existing throughout the duct, from its opening at the nipple 

 to the secreting culs-de-sac. The internal coat is an amorphous membrane, 

 lined with flat, polygonal cells during the intervals of lactation and even dur- 

 ing pregnancy, the cells being cylindrical in form and frequently presenting 

 multiple nuclei, when milk is secreted. 



The acini of the gland, which are very abundant, are visible to the naked 

 eye, in the form of small, rounded granules of a reddish-yellow color. Be- 

 tween these acini, there exists a certain quantity of the ordinary white fibrous 

 tissue, with quite a number of adipose vesicles. The presence of adipose 

 tissue in considerable quantity in the substance of the glandular structure is 

 peculiar to the mammary glands. Each acinus is made up of twenty to 

 forty secreting vesicles. These vesicles are irregular in form, often varicose, 

 and sometimes they are enlarged and imperfectly bifurcated at their termi- 

 nal extremities. During lactation their diameter is -^ to -^ of an inch 

 (60 to 80/t). 



During the intervals of lactation, as the lactiferous ducts become re- 

 tracted, the glandular culs-de-sac disappear ; and in pregnancy, as the gland 

 takes on its full development, the ducts branch and extend themselves, and 

 the vesicles are gradually developed around their extremities. 



Mechanism of the Secretion of Milk. With the exception of water and 

 inorganic matters, all the important and characteristic constituents of the 

 milk are formed in the substance of the mammary glands. The secreting 

 structures have the property of separating .from the blood a great variety of 

 inorganic salts ; and the milk furnishes all the inorganic matter necessary 

 for the nutrition of the infant, even containing a small quantity of iron. 



The lactose, or sugar of milk, the caseine, 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, in- 

 jected 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. 



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

 transformation of the albuminoid constituents of the blood. The fatty par- 

 ticles 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 glands is 

 somewhat obscure. The particles are produced in the cells, probably by a 

 process analogous to that which takes place in the formation of the fatty 

 particles found in the sebaceous matter. 



As regards the mechanism of the formation of the peculiar and character- 

 istic constituents of the milk, the mammary glands are to be classed among 

 the organs of secretion and not with those of elimination or excretion ; for 

 none of these elements pre-exist in the blood, and they all appear first in the 

 substance of the glands. 



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



