820 THE SECRETION OF MILK. [BOOK n. 



lactation. In both sexes this initial activity soon passes off, the 

 gland in the female further developing at puberty, but in the male 

 remaining, save in exceptional cases, in its infantile condition or 

 somewhat retrograding. 



516. The secretion of milk. From what has been already 

 said it is obvious that the secretion of milk, while resembling 

 the secretion of the other secreting glands which we have studied 

 in being essentially an activity of the epithelium cells lining the 

 alveoli, nevertheless presents certain interesting features special to 

 itself. If the account given in 510 be a true one, morphological 

 changes in the cells are more prominent than in the case of other 

 glands ; and we may interpret the appearances there related some- 

 what as follows. When the discharged gland with its low epithelium 

 begins the work of loading, the cells distinctly ' grow.' Their cell- 

 substance increases in bulk, and elongating projects into the lumen 

 of the alveolus. At the same time the nucleus divides as if the 

 cell were about to give birth to new cells ; but at first at all events 

 no division of the cell-substance takes place, and the new nuclei lie 

 imbedded in a common cell body. The cell-substance meanwhile 

 puts on secretory activity; it deposits in itself material to form 

 milk. The deposit of fat is conspicuous and easily recognised, 

 but we may fairly infer that the other less easily distinguished 

 proteid and carbohydrate materials are deposited in the cell- 

 substance in a similar fashion. Then follows the ejection of the 

 prepared material ; and this may take place in one of two ways. 

 The oil globules of fat may be protruded from the cell-substance 

 much in the same way that an amoeba extrudes its excrement, and 

 possibly other constituents of milk may be ejected by a similar 

 method. But besides this, the deferred cell division now takes 

 place in a somewhat imperfect fashion, so that portions of the 

 old cell carrying nuclei with them come asunder from the rest of 

 the cell in which a nucleus is left, and lie loose in the lumen of 

 the alveolus; portions of cell-substance free from nuclei appear 

 also to be cast off. Here, in the lumen of the alveolus, they 

 rapidly undergo change; the cell-substance is altered and dis- 

 solved, and its load of prepared material, probably undergoing in 

 the act some further change, is set free, the nuclei also under- 

 going change and becoming ultimately broken up. Hence the 

 constituents of milk are provided for, not only as in other glands 

 by the material with which the cell loads itself and subsequently 

 discharges into the lumen of the alveolus, but also by the actual 

 substance of part of the cell itself. We may perhaps infer that 

 in consequence of this intervention of the actual cell-substance 

 and nucleus in the formation of the milk the main proteid of milk 

 is the characteristic caseinogen with its nuclein constituent and 

 not one of the more ordinary proteids. 



It is hardly necessary to add that these bodily contributions 

 of the secreting cell to the secretion are accompanied by that 



