822 THE SECRETION OF MILK. [BOOK n. 



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. 



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 



aible from any kind of sugar or glycogen. A glycogen-like 

 j 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 for- 

 mation, 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 

 in some way or other, we should have an image of the nutrition of 

 such a tissue as muscle, in which the proteid constituent is in 

 excess of the others. 



518. That both the secretion and ejection of milk are under 

 the control of the nervous system is shewn by common experience, 

 but the exact nervous mechanism has not yet been fully worked 

 out. While 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 sympathetic as well as the spinal 

 nerves are cut. 



