LACTATION 



591 



Three different hypotheses have heeii put forward regarding the 

 manner in wlrioh the substances of which the milk is formed pass out 

 from the secretory cells. According to one view, the cells themselves 

 break loose and liecome 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 Ijecoming detached or destroyed 

 themselves, as with the submaxillary mucous gland. According to 

 the third hypothesis the mammary gland in its mode of activity 



lfi4. 



— Sectioji of mamiuai'V ghmd (huiuau) in full activity. 

 (From Sharpey iScbafer, after von Ebner.) 



<(, c(', a". Alveoli variously cut and distended by secretion ; y, ;/, commencing 

 ducts ; /, connective tissue. 



occupies a position midway lietween the seliaceous and sulmiaxillary 

 glands ; some of the cells simply discliarging their contents into the 

 lunnna, while with others, the central part of the cell, containing a 

 degenerate daugliter nucleus, breaks awa}' and liecomes disintegrated, 

 leaving the liasal portion still in position. 



It has already Ijeen menti(.)ned that tlie mammary glands of all 

 Mammals, with the exception of the Alonotremes, 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 



