464 NUTRITION OF FARM ANIMALS 



reach their full development only in the later stages of preg- 

 nancy. At that time, a rapid growth of the alveoli and per- 

 haps the formation of new ones occurs, the stimulus to this 

 growth being, according to Bayliss and Starling, the formation 

 of certain stimulating substances (Hormones) in the fetus which 

 .pass into the blood of the mother and so reach the milk glands. 

 That other causes may at least cooperate, however, is shown 

 by the apparently well-established fact that the regular re- 

 moval of the fluid found in the glands of the virgin animal, 

 or even mechanical stimulation, may lead to the formation 

 of considerable quantities of milk, in some instances even in 

 the male. 



550. The secretion of milk. That milk formation is a true 

 secretion and not a mere nitration of material from the blood 

 is clearly shown by the facts already stated regarding the com- 

 position of milk. As was pointed out, all the principal organic 

 ingredients of the milk are peculiar to it. Casein and lactose 

 are not found elsewhere in the animal body, and while the prin- 

 cipal simple fats of milk are also found in the body fat, their 

 proportions are different in the milk fat and the latter is specially 

 characterized by the presence of glycerids of the lower acids 

 of the aliphatic series. Furthermore, even more marked quan- 

 titative differences exist between the mineral elements of the 

 milk and those of the blood serum. From all these facts, it is 

 clear that the milk gland is a producing or secreting organ and 

 that the solid ingredients of the milk are largely manufactured 

 in it out of materials derived from the blood. 



A theory of milk secretion first propounded by Virchow 

 found wide acceptance. According to this theory, milk pro- 

 duction consists essentially of a physiological fatty degeneration 

 of the epithelial cells of the alveoli. The microscope shows that 

 the cells of the actively secreting gland are larger than those 

 in the resting gland and more or less filled with fat globules, 

 especially on the side toward the cavity of the alveolus. It was 

 held that while this process went on the cell divided, forming 

 two or more, and that finally the cell next to the cavity liquefied, 

 setting free the fat globules which it contained' and, perhaps 

 with the addition of more or less water, constituted the milk. 

 Milk production was thus regarded as a form of the growth of 

 tissue. 



