616 THE SECKETION OF MILK. [BOOK n. 



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. 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 consequence of this 

 intervention of the actual cell-substance in the formation of the 

 milk. 



The secretion of milk differs from such a secretion as that 

 of saliva, and approaches the formation of sebum inasmuch 

 as the transformed cell-substance is shed bodily to form part of 

 the milk. We say form part of the milk because this gross mode 

 of secretion is accompanied by the more ordinary mode. The 

 cells are at the same time in the more ordinary way discharging 

 into 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. Per- 

 haps however we ought not to call it a double mode, for the 

 one method really passes insensibly into the other. The dis- 

 charge 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 dif- 

 ferent phases of the same act of secretion. 



411. The secretion of milk then would appear to illus- 

 trate, 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. Experimental 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 contrary diminished, by fatty food. This effect on the 



