SECRETION OF MILK 



| of secreting cells. When the gland is inactive and not 

 [secreting milk these cells are cubical or somewhat 



flattened, and contain granular protoplasm with a 

 .single oval nucleus (Fig. 39). When milk is being 

 'produced they under- 

 jgo peculiar changes. 



They increase in 



length very con- 

 siderably and extend 



into the cavity of the 



alveoli, thus reduc- 

 ing its empty space. 



Each cell may now 



possess two or more 



nuclei, the basal one 



normal in form and 



size, the other near 



the free end of the 



cell being more or 



leSS degenerate and FlG< 39 ._Transverse sections of alveoli 01 milk glands 



irregular in outline 

 and structure. With- 

 in the cells are pro- 

 duced numbers of globules of oil or fat, as well as 

 other materials, and from them water, salts of various 

 kinds, sugar, and probably some of the oil, is dis- 

 charged into the cavity of the alveolus and tend 

 to fill it. Sooner or later the elevated portions of 

 the secreting cells, with their degenerate nuclei, pro- 

 toplasmic contents and oil, are cast off into the 

 I cavity of the alveoli and float in the liquid already 

 secreted into it, the whole mixture forming what is 

 known as milk. The detached solid parts of the cells 



of cow. 



1. Inactive alveolus. 



2. Alveoli during milk secretion ; a. secreting epi- 



thelium ; b fat globule in cavity of alveolus. 

 ( X 300). 



