LACTATION 591 



Three different hypotheses have been put forward regarding 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 aa 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 



FIG. 164. Section of mammary gland (human) in full activity. 

 (From Sharpey Schafer, after von Ebner.) 



, ',"", Alveoli variously cut and distended by secretion ; g, g\ commencing 

 ducts ; ?, connective tissue. 



occupies a position midway between the 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 



