LACTATION 559 



the manner in which the substances of which the milk is formed 

 pass out from the secretory cells. According to one view, the 

 cells themselves break loose and become disintegrated, setting 

 free their contents in the alveoli of the gland, just as in the case 

 of the sebaceous glands. 



Another theory states that the cells simply excrete the 

 substances into the alveolar lumina without becoming detached 

 or destroyed themselves, as with the submaxillary mucous gland. 

 According to the third hypothesis the mammary gland in its 

 mode of activity occupies a position midway between the 



Fig. 138. — Section through an alveolus with fat drops in cells. (From 

 Schafer, after von Ebner.) 



e, cells of alveolus ; k, cells of basement membrane (m) ; i, connective tissue. 



sebaceous and submaxillary glands ; some of the cells simply 

 discharging their contents into the lumina, while with others,, 

 the central part of the cell, containing a degenerate daughter 

 nucleus, breaks away and becomes disintegrated, leaving the 

 basal portion still in position. 



It has already been mentioned that the mammary glands of 

 all Mammals, with the exception of the Monotremes, are usually 

 regarded as being of the nature of modified sebaceous glands. 

 It was partly on account of this belief that certain of the older 

 writers held the view that the secretion of milk was the result 

 of a fatty degeneration leading to a complete disintegration of 

 the secretory cells of the mammary gland.'^ According to this 



' Virchow, Die Cellular-Pathologie, Berlin, 1871. 



