2(34 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



He found that artificial stimulation of sensory nerves causes a diminution in 

 the amount of secretion, thus confirming the opinion based upon observations 

 upon the human being, that in some way the central nervous system exerts an 

 influence on the mammary gland. When the mammary glands are com- 

 pletely isolated from their connections with the central nervous system, stimu- 

 lation of an afferent nerve no longer influences the secretion. Mironow 

 states also that although section of the external spermatic on one side does not 

 influence the secretion, section of this nerve on both sides is followed by a 

 marked diminution, and the same result is obtained when the gland on one 

 side is completely isolated from all nervous connections. The diminution of 

 the secretion in these cases comes on very slowly, after a number of days, so 

 that the effect cannot be attributed to the removal of definite secretory fibres. 

 Moreover, after apparently complete separation of the gland from all its 

 extrinsic nerves, not only does the secretion, if it was previously present, con- 

 tinue to form although in less quantities, but in operations of this kind upon 

 pregnant animals the glands increase in size during pregnancy and become 

 functional after the act of parturition. 



Experiments, therefore, as far as they have been carried, indicate that 

 the gland is under the regulating control of the central nervous system, either 

 through secretory or vaso-motor fibres, but that it is essentially an automatic 

 organ. The bond of connection between it and the uterus seems to be, in part 

 if not entirely, through the blood rather than through the nervous system. 

 It should be added that Arnstein 1 has described a definite connection between 

 the nerve-fibres and the epithelial cells of the gland. If this fact is corrobo- 

 rated it would amount to an histological proof of the existence of special 

 secretory fibres, but the physiological evidence for the same fact is either 

 negative or unsatisfactory. 



Normal Secretion of the Milk. — As was said in speaking of the his- 

 tology of the gland, the secreting alveoli are not fully formed until the first 

 pregnancy. During the period of gestation the epithelial cells multiply, the 

 alveoli are formed, and after parturition secretion begins. At first the secre- 

 tion is not true milk, but a liquid differing in composition and known as the 

 colostrum ; this secretion is characterized microscopically by the existence of 

 the colostrum corpuscles, which seem to be wandering cells that have under- 

 gone a complete fatty degeneration. After a few days the true milk is formed 

 in the manner already described. According to Rohrig the secretion is con- 

 tinuous, but this statement needs confirmation. As the liquid is formed it 

 accumulates in the enlarged galactophoraus ducts, and after the tension has 

 reached a certain point further secretion is apparently inhibited. If the ducts 

 are emptied, by the infant or otherwise, a new secretion begins. The emptying 

 of the ducts, in fact, seems to constitute the normal physiological stimulus to 

 the gland-cells, but how this act affects the secreting cells, whether reflexly or 

 directly, is not known. When the child is weaned the secretion under normal 

 conditions soon ceases and the alveoli undergo retrograde changes, although 

 1 Anatomischer Anzeiger, L895, Bd. x. S. 410. 



