CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 821 



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. And 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 

 histological 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. 



The casein in a similar way seems to be formed by the action 

 of the cell. It cannot be gathered out of the blood, since the blood 

 contains no real casein ; it must be formed in the gland. Some 

 observers have maintained that when milk is kept at 35, the 

 casein is increased through some ferment action taking place in 

 the milk itself; but this seems not to be the case, and the for- 

 mation of casein must be regarded as the result of the action of 

 the cell. Even the albumin present appears to be not the ordinary 

 serum-albumin simply passed from the blood through the cell into 

 the lumen of the alveolus, but the slightly different lactalbumin. 

 We may perhaps regard the albumin as less difficult to manufacture 

 than the casein ; and we may explain the fact that relatively to 



