GENERATION AND DEVELOPMENT 727 



ance of the apex or free border being separated from the base 

 by a process of constriction. If the gland be examined when 

 discharged — viz., after the milk has been drawn off — the cells 

 lining the alveolus are cubical or flattened, each containing 

 a nucleus ; the lumen of the alveolus is also increased in size, 

 and within it may be seen some of the elements of the milk 

 (Fig. 257). It is evident that the cells in the active gland are 

 loaded with material, much of it being fat, and these cells break 

 off, leaving behind them the parent cell, containing a nucleus 

 from which another cell grows. In spite of this, the formation 

 of fat in milk is really a process of cell secretion, and this is 

 supported by the fact that animals such as carnivora, whose 

 food is deficient in fat, produce a fat-containing milk, the fat 

 being elaborated by the mammary cell from the protein of the 

 body. A fat diet does not increase the fat in milk, though a 

 protein diet has this effect. 



The proteins, sugar, and salts, found in milk are secreted in 

 the ordinary way from the blood, or rather the lymph, circu- 



Loaded. Discharged. 



Fig. 257. — Mammary Gland of Dog during Lactation. After 

 Heidenhain (Waller). 



lating in the gland, the cells lining the alveolus being the active 

 factor in the matter. That these substances are really elaborated 

 by the cell is supported by the fact that neither caseinogen nor 

 milk sugar exists in any other tissue of the body. It has been 

 supposed that the secretion of milk is influenced by the nervous 

 system, but there is no experimental evidence which places this 

 beyond doubt. 



Composition. — The milk of herbivora has an alkaline reaction 

 which may readily turn acid. In carnivora the reaction is acid. 

 Fresh cow's milk is amphoteric — viz., it gives both an acid and 

 an alkaline reaction to test-paper. This is due to the presence 

 of acid and alkaline salts. In the cow the specific gravity is 

 1028 to 1034. The secretion contains proteins (caseinogen 

 and albumin), sugar (lactose), fats, and salts. An average 

 secretion of milk from a cow may be taken at 6 quarts (6-8 litres) 

 per diem for forty weeks in the year. Assuming this to contain 



