704 REPRODUCTION AND GROWTH 



are fully formed in the boy as well as in the girl at birth, and up to the end 

 of the second to eighth week after birth they secrete a milky fluid called 

 " witch's .milk/' In males the mammary glands as a rule become only slightly 

 developed and never produce any secretion. In the female they begin at 

 puberty to grow and increase considerably in size at that time. But a really 

 important increase takes place only in connection with pregnancy. During 

 the last few weeks of gestation, a fluid which differs considerably in com- 

 position from the milk and is known as the colostrum., is given off, and after 

 birth has occurred the glands enter upon a period of vigorous activity which, 

 with the child at the breast, continues for months. When the child is not 

 nursed by the mother, the mammary glands atrophy and shrivel up to a 

 small mass of connective tissue. 



The nerves of the mammary glands end about the gland cells as a net of 

 cirrous arborizations. In the human species they arise from the fourth to 

 sixth intercostal nerves. 



In animals the mammary glands are situated more distally and accordingly 

 are supplied by more distal nerves. In the guinea pig, which has only a single 

 pair of mammary glands, the nerves come from the spermatics. The five pairs 

 of glands in the dog receive their nerves from this and several other nerve 

 trunks. 



The influence of the' nervous system on the secretion of milk is a subject 

 which has not yet been sufficiently investigated. From the observations of 

 Goltz and Ewald on animals with the lower end of the spinal cord exsected 

 (page 583), we learn that the mammary glands can secrete milk independ- 

 ently of the central nervous system. But the milk secreted by glands whose 

 nerves have been sectioned exhibits morphological changes which go to show 

 that the secretion is influenced in certain ways by the nervous system (K. 

 Basch). Besides in the above experiments of Goltz and Ewald the influence 

 of the peripheral sympathetic ganglia was not excluded. 



Concerning the mechanism of milk secretion, the most divergent views 

 have been expressed, and for the present we cannot decide whether the gland 

 cells become disintegrated and contribute their own substance to the secretion, 

 or whether they prepare the constituents of the milk from other materials. 

 It seems, however, that there can be no very extensive destruction of cells, 

 and the real question therefore narrows down to whether or not nuclei and 

 protoplasm become to some extent transformed into secretion. Most recent 

 writers on the subject think that this is the case ; but that only the free end 

 of the gland cell is lost, and that after the secretory products have in this 

 way been discharged, a regeneration of the protoplasm from the basal, nucle- 

 ated end of the cell takes place, and the same process of transformation, 

 disintegration, etc., is repeated. For more detailed information we must refer 

 to the literature bearing on the subject. Here we may only add that during 

 lactation a varying number of leucocytes wander out of the interstitial con- 

 nective tissue into the alveoli of the glands. It is possible that in the event 

 of arrested lactation, they convey the fat away from the mammary gland. 



Neither casein nor milk sugar is found in the blood ; hence it is evident 

 that they must be formed in the mammary gland itself. 



