MAMMAKY SECKETION. 627 



replaced by oil-globules, which entirely agree in their characteristics with 

 the oil-globules found in milk. So, also, in feeding animals on highly 

 albuminous diet they may even be seen to increase in weight, while at 

 the same time more fat is removed in the milk than could be taken in the 

 food, while the increase in weight indicates that the origin of fat is not 

 from the adipose tissue of the bod}^. On the other hand, it is impossible 

 that in the herbivora the fat of the milk should be derived from the break- 

 ing up of albuminoids in the gland, for the total amount of albuminoids 

 breaking up in the body is insufficient to furnish the fat removed in the 

 mammary secretion. Part must, therefore, be derived from the blood. 



As regards the casein, this substance is, without doubt, developed 

 at the expense of the albuminous cell-contents, since it is absent from the 

 blood, the alkali albuminate being directly derived from the breaking 

 down of the protoplasm, while the nuclein, which is to be regarded as a 

 constant component part of the casein, is, without doubt, derived from 

 the nucleus which disappears in the process of secretion. This con- 

 version of the albuminous contents into casein is still further evidenced 

 by the fact that the proportion of casein in the milk depends upon the 

 degree of perfection of cell activity. Thus, in the earlier stages of 

 lactation, in the formation of colostrum, the amount of albuminoid 

 matter contained in the milk is greatly in excess of the amount contained 

 in milk after lactation has become thoroughly established, while coinci- 

 dently with the decrease in albumen there is a proportionate increase 

 in the percentage of casein. A ferment has even been extracted from 

 the mammary gland which possesses the power of converting albumen 

 into casein. 



The origin of milk-sugar is less clearly established, although it also 

 seems, without doubt, to originate in changes occurring in the proto- 

 plasmic contents of the epithelial cells of the mammary gland. For the 

 amount of sugar in the milk is entirely independent of the amount of 

 carbohydrate constituents of the food, and remains unchanged even 

 when animals are fed on a purely meat diet. It would, therefore, appear 

 that the milk-sugar, casein, and fats are all formed by the direct activitv 

 of the epithelial cells as a result of the decomposition of their proto- 

 plasmic contents or their action on the food-constituents in the blood. 



The other constituents of the milk, the water and salts, evidently 

 result from a direct process of transudation from the blood, with the 

 exception that, without doubt, a certain percentage of the potassium 

 salts and phosphates, like the specific milk-constituents, originate in the 

 metamorphosis of the protoplasm of the secretory cells. 



The process of secretion of milk may, therefore, be regarded as a 

 process of molting of the epithelial cells, which undergo decomposition 

 and discharge the resulting products into the excretory ducts. 



