506 PHYSIOLOGY CHAP. 



fats are there in minute quantities only, whereas they are 

 abundant in milk ; lactose is not found at all in blood ; lastly, 

 the inorganic constituents of milk and blood are quantitatively 

 different. 



The yield of milk from the body in 24 hours is generally 

 considerable. During well-established lactation a good wet-nurse 

 will produce a litre or a litre and a half per diem. This amount 

 may be greatly increased since it depends on a number of 

 factors, the principal of which is the degree of development of 

 the mammary gland. Interesting observations on this subject 

 have been made on the cow. Two cows of the same breed and 

 approximately the same weight produce very different quantities 

 of milk, in proportion with the development of the udder. Accord- 

 ing to Fleischmann the maximal yield from a milch cow is 24 

 litres = 25 kilos, milk per diem, with about 3 kilos, solid substances. 



Since the maximal weight of the udder is about 5 kilos, with 

 1'5 kilos, solid substances, it follows that in these extreme cases 

 the udder secretes five times its own weight in the day. 



The development of the gland, which is maximal during the 

 first weeks after parturition, diminishes with the duration of 

 lactation, and the yield of milk decreases in proportion. The 

 influence of race upon the development of the udder and con- 

 sequently of its yield of milk is great. The best milch cows are 

 those of the Swiss, Dutch, and Oldenburg breeds. According to 

 Fleischmann muscular work, within certain limits, conspicuously 

 diminishes the lacteal secretion, and in time determines a certain 

 amount of glandular involution. This is why the race of cattle 

 employed in heavy agricultural labour give little milk. 



VIII. In order to form an idea of the process by which the 

 mammary gland forms the specific constituents of milk, which do 

 not pre-exist in blood and lymph, we must examine the effect of 

 nutrition in general, and of specific diets in particular, upon 

 lacteal secretion. 



It is only necessary to compare the amount of organic 

 substances contained in milk with a scanty and an abundant diet, 

 to see the great influence of alimentation in general upon the 

 functional activity of the mammary gland. Every one knows 

 that a wet nurse must be well fed in order to have good milk, 

 which in physiological language means that the crude materials 

 furnished by the blood and lymph to the mammary gland for the 

 production of milk come for the most part directly from the food. 

 It is, however, far more important to see how the chemical com- 

 position of milk alters with a preponderance in the diet of protein, 

 fat, or carbohydrate. A priori one would suppose that there 

 would be a ratio between the quantity of protein, fat, and carbo- 

 hydrate ingested and the amount of the same in the milk. 

 Experiment shows, on the contrary, that a diet rich in protein not 



