ix THE SKIN AND CUTANEOUS GLANDS 507 



only improves the yield of milk as a whole, but also the amount 

 of its principal constituents, particularly the fat content. 



This fact was established in 1846 by Franz Simon, for nursing 

 women, and confirmed by Decaisne in 1873. Still more striking 

 results were obtained by Ssubotin (1866), on comparing the 

 chemical composition of the milk of a bitch fed on meat with one 

 fed on potatoes only. 



The fact that a full flesh diet largely increases the fat content 

 of milk supplied a valid argument in favour of the theory specially 

 sustained by Voit, that animal fat is mainly derived from protein. 

 The secretory cells of the mammary gland principally employ 

 protein as the material from which they elaborate the organic 

 constituents of milk. A full protein diet first develops or increases 

 the secreting cells and the size of the gland, and then the total 

 quantity of the secretion and its fat content increase also. The 

 effect is not seen at once, but after a few days, and it is more 

 pronounced in the early months of suckling than in the later, 

 when the gland begins to undergo a slow process of involution. 

 Once the gland has been developed, a smaller amount of alimentary 

 protein suffices to maintain its increased function. 



The effect of a protein diet upon the sugar content of milk 

 has been much disputed. According to the experiments of 

 Ssubotin on bitches, and of Kiihn on cows, the lactose is somewhat 

 diminished ; but according to I. Munk's later experiments (1881), 

 full protein feeding augments not only the fat and protein content 

 of milk, but the amount of sugar also. 



A full diet of fats not only does not increase the fat content 

 of milk but even diminishes it, if the food does not contain a 

 sufficient amount of protein at the same time. In the latter case 

 there is an increase of butter such as occurs on a richly nitrogenous 

 diet. From this it may be deduced that the ingested fat does not 

 tend to increase the fat of the milk, but, by sparing the nitrogenous 

 consumption, makes it possible for the protein to be utilised more 

 largely for the formation of butter, as shown by the admirable work 

 of Pettenkofer and Voit, to which we shall have to refer below 

 in speaking of general metabolism. That increase of alimentary 

 fat, with a constant amount of alimentary protein, does not increase 

 the amount of butter in the milk, is directly proved by Kiihn's 

 investigations with cows. 



The amount of carbohydrate fed has no appreciable effect on 

 the amount of lactose secreted by carnivora. But even in herbi- 

 vora no relation can be observed between the quantity of ingested 

 starch and the sugar content of the milk. Since, as a rule, a 

 starchy diet coincides with scarcity of protein food, it follows that 

 the secretory cells of the gland find little material to hand for the 

 formation of the milk as a whole. That not only the fat but also 

 the lactose (for the most part at any rate) come from the protein 



