368 SECRETION. 



pose 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 from twenty to forty secreting vesicles, or culs-de-sac. These vesicles are irregular in 

 form, often varicose, and sometimes they are enlarged and imperfectly bifurcated at their 

 terminal extremities. During lactation, their diameter is from ^ to -^ of an inch. Dur- 

 ing pregnancy, and when the gland has just arrived at its full development, the secreting 

 vesicles are formed of a structureless membrane, lined with small, nucleated cells of 

 pavement-epithelium. The nuclei are relatively large, ovoid, and are embedded in a small 

 amount of amorphous matter, so that they almost touch each other. Sometimes the epi- 

 thelium is segmented, and sometimes it exists in the form of a continuous nucleated 

 sheet. When the secretion of milk becomes active, the epithelium entirely disappears, 

 and it reappears as the secretion diminishes. This observation is due to Robin and has an 

 important bearing upon the mechanism of the secretion of milk. 



During the intervals of lactation, as the lactiferous ducts become retracted, the glan- 

 dular culs-de-sac disappear ; and, in pregnancy, as the gland takes on its full develop- 

 ment, the ducts branch and extend themselves, and the vesicles are gradually developed 

 around their terminal extremities. These changes in the development of the mamma3 at 

 different periods are most remarkable and are not observed in any other of the glandu- 

 lar organs. 



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

 principles, 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 sep- 

 arating from the blood a great variety of inorganic principles ; and we shall see, when 

 we come to study the composition of the milk more minutely, that it furnishes all the 

 inorganic matter necessary for the nutrition of the infant, containing, even, a small quan- 

 tity of iron. Precisely how the secreting vesicles separate the proper quantity of these 

 principles from the circulating fluid, we are unable, in the present state of our knowl- 

 edge, to state. 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 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 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 peculiar transformation of 

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

 although its presence here has been mentioned by some observers. 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. 



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 glands is obscure. The particles 

 are not produced in cells and set free by their rupture, by a process analogous to that 

 which takes place in the formation of the fatty particles found in the sebaceous matter, 

 for, during the time when the secretion of milk is most active, the epithelium of the 

 secreting culs-de-sac has entirely disappeared. The butter is produced by the action of 

 the amorphous walls of the vesicles, in the same way, probably, as fat is produced by 

 the vesicles of the ordinary adipose tissue. At least, this is all that is known regarding 

 the mechanism of its production. 



