THE MAMMARY GLAND. 481 



430. 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 epithelial cells lining the alveoli, nevertheless presents certain inter- 

 esting features special to itself. If the account given in 424 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 in- 

 creases 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 recognized, but 

 we may fairly infer that the other less easily distinguished proteid and car- 

 bohydrate 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 extruded 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 dissolved, and its 

 load of prepared material, probably undergoing in the act some further 

 change, is set free, the nuclei also undergoing change and becoming ulti- 

 mately 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. The characteristic nuclein of the 

 milk has thus its origin in all probability in the shed nuclei of the secreting 

 cells, and we may perhaps infer that the still more characteristic casein 

 exists in milk in the form of casein and not of some other proteid in conse- 

 quence of this intervention of the actual cell substance in the formation of 

 the milk. 



431. The secretion of milk then would appear to illustrate, even more 

 fully and clearly than do other glands, the truth on which we have so often 

 insisted, that a secretion is eminently the result of the metabolic activity of 

 the secreting cell. The blood is the ultimate source of milk, but it becomes 

 milk only through the activity of the cell, and that activity consists largely 

 in a metabolic manufacture by the cell, and in the cell, of the common things 

 brought by the blood into the special things present in the milk. Experi- 

 mental results tell the same tale. Thus the quantity of fat present in milk 

 is largely and directly increased by proteid, but not increased on the con- 

 trary diminished by fatty food. This effect on the mammary gland in 

 particular is in accordance with what we shall presently learn to be the 

 general effect on the body of proteid in contrast to that of fatty food ; 

 proteid food seems to increase the general metabolic activity of the body, 

 while fatty food tends to lessen it. Moreover the proteid food seems actually 

 to furnish the fat ; and we have already suggested a manner in which pro- 

 teids may give rise to fat. That the fat of the milk need not necessarily 

 come from the fat of the food is shown by the following experiment : A. bitch 



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