CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 785 



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. 



It is hardly necessary to add that these bodily contributions of 

 the secreting cell to the secretion are accompanied by that more 

 ordinary part of secretion which consists in the flow of fluid 

 containing various matters in solution through the cells into the 

 alveoli, the general composition of the milk being thus secured. 



517. 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. 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 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 proteids may give rise to fat. That the fat of the milk 

 need not necessarily come from the fat of the food is shewn by the 

 following experiment. A bitch fed on meat for a given period 

 gave off more fat in her milk than she could possibly have taken 

 in her food ; and this moreover took place while she was gaining 

 in weight and ' laying on fat,' so that she could not have supplied 

 the mammary gland with fat by simply transferring fat from the 

 store previously existing in the adipose tissue of her body; she 

 apparently obtained the fat ultimately from the proteids of her 

 food. Arid the histological facts given above favour the view that 

 the formation of fat out of proteids in such cases takes place in the 

 cells of the alveoli. The experimental then as well as the histo- 

 logical evidence goes to shew that the fat of milk is formed in the 

 cell and by the cell, and is not simply gathered out of the blood. 



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