786 THE SECRETION OF MILK. [BOOK 11. 



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 presence in milk is not dependent on carbo- 

 hydrate 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 forma- 

 tion, by the metabolic activity of the secreting cell, of the 

 representatives of the three great classes of food-stuffs, proteids, 

 fats, and carbohydrates. It is of course quite true that all the cell 

 has to do may be simply to turn aside into the special casein, fats, 

 and lactose, the general supply of proteids, fats, and carbohydrates 

 brought to it in the blood, without these ever becoming actually 

 part of the cell, the formation of fat out of proteid spoken of 

 above taking place in some other part of the body. Still it is 

 open for us to suppose that they are all three formed in the cell 

 itself out of the comprehensive living cell-substance. If we 

 accept the latter view we may look upon what is taking place in 

 the mammary cell as a picture of what is going on in various living 

 tissues. If the fat of the milk were not ejected from the 

 mammary cell, the mammary gland would become a mass of 

 adipose tissue, especially if, by a slight change in the metabolism, 

 the production of fat were exalted at the expense of the production 

 of casein or milk-sugar. If, again, by a similar slight change the 

 milk-sugar were accumulated rather than the fat or proteid, we 

 should have a result which, by an easy step, would bring us to 

 glycogenic tissue. And, lastly, if the proteid accumulation were 

 greater than the fatty, or the saccharine, these being carried off 



