816 SECRETORY CHANGES IN MAMMARY GLAND. [BOOK n. 



the lumen water holding saline and other constituents in solution. 

 And the peculiar features of milk, as we shall see presently, 

 correspond to this double mode of secretion. Perhaps however 

 we ought not to call it a double mode, for the one method really 

 passes insensibly into the other. The discharge of sodium chloride 

 in solution from every kind of gland, of mucin from a mucous 

 gland, of oil globules with a proteid envelope from a mammary 

 gland, and lastly of nucleated loaded cell-substance from the 

 mammary gland, present so many different phases of the same 

 act of secretion. 



511. The dormant resting mammary gland, that for instance 

 of an animal which has never been pregnant, is much smaller than 

 a suckling gland, owing to the alveoli being both smaller and less 

 numerous. Each alveolus moreover is not a cavity lined with 

 a single layer of epithelium, but a solid cylinder or mass of 

 comparatively small, rounded or polyhedral cells. So long as 

 pregnancy does not occur the growth of these is exceedingly slow, 

 and the products of such metabolism as goes on in them are 

 carried away by the blood, so that under normal circumstances no 

 secretion takes place. 



When pregnancy occurs rapid growth of the mamma takes 

 place, numerous new alveoli being formed by budding, but all for 

 a time remaining solid cylinders of cells. At the approach of the 

 birth of the offspring, the central cells undergo metabolic changes, 

 especially a fatty transformation, and either before or after birth 

 are cast off, leaving a single layer to line the alveoli and to carry 

 on the work of secretion as described above. It is generally 

 supposed that these shed cells supply the so-called ' colostrum 

 corpuscles ' characteristic of the first milk, of which we shall speak 

 presently. At the end of lactation an absorption of some of the 

 alveoli takes place ; and in old age still further absorption goes on 

 with great diminution of the lumina. 



512. The connective tissue, joining together the lobules of 

 various sizes, surrounding the lobules and running in between the 

 projecting blind ends of the alveoli within the lobules, is rich in 

 blood vessels which form capillary networks round the alveoli ; it 

 also carries a considerable number of lymphatic vessels which arise 

 in lymph-spaces around the alveoli and elsewhere. Leucocytes 

 are numerous in the spaces of this connective tissue, and some of 

 them may make their way through the basement membrane and 

 between the secreting cells into the cavities of the alveoli and so 

 appear in the milk. 



513. The nature of milk. Human milk has a specific gravity 

 of from 1*028 to 1*034, and when quite fresh possesses a slightly 

 alkaline reaction. It speedily becomes acid ; and cow's milk, even 

 when quite fresh, is sometimes slightly acid, the change of reaction 

 taking place during the stagnation of the milk in the mammary 

 ducts. 



