Chap, iv.] METABOLIC PROCESSES OF THE BODY. 617 



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 histo- 

 logical 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 evi- 

 dence 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 formation 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 ex- 

 plain the fact that relatively to the albumin the casein is less at 

 the very beginning and especially toward the end of lactation, 

 by supposing that the cell has in the first case not got into full 

 working order and in the second case is waning in power. The 

 peptone-like body in milk though small in quantity is a further 

 indication of the proteid metabolism taking place in the cell. 



That the milk-sugar, lactose, also is formed in and by the 

 cell, is indicated by the facts that it is found in no other part 

 of the body, and that its presence in milk is not dependent on 

 carbohydrate food, for it is maintained in abundance in the 

 milk of carnivora when these are fed exclusively on meat, as 

 free as possible from any kind of sugar or glycogen. A glyco- 

 gen-like body has moreover been described as existing in the 

 cells, and it is suggested that this body is the antecedent of 

 the lactose. We thus have evidence in the mammary gland 



