482 NUTRITION. 



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 pre- 

 viously 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 favor 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 show 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 

 explain 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 pres- 

 ence 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 glycogen-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 of the formation, by the 

 metabolic activity of the secreting cell, of the representatives of the three 

 great classes of food-stuffs, proteids, fats, and carbohydrates. 



432. That both the secretion and ejection of milk are under the con- 

 trol of the nervous system is shown by common experience, but the exact 

 nervous mechanism has not yet been fully worked out. While the erection 

 of the nipple ceases when the spinal nerves which supply the breast are 

 divided, the secretion continues, and is not arrested even when the sym- 

 pathetic as well as the spinal nerves are cut. 



CHAPTEE V. 



NUTRITION. 

 THE STATISTICS OF NUTRITION. 



433. THE preceding chapter has shown us how wholly impossible it is 

 at present to master the metabolic phenomena of the body, by attempting to 

 trace out, forward or backward, the several changes undergone by the indi- 

 vidual constituents of the food, the body, or the waste products. Another 



