LACTATION 



589 



In the secretory cells of the lactating mammary gland an active 

 and a resting condition can be distinguished. In the latter the 

 lumina of the alveoli are wide, and the cells of the lining epithelium 

 form a single flat layer with centrally situated nuclei. In the active 

 condition the epithelial cells are long and columnar, and project into 

 the lumina, and some of them have two nuclei. 



In these cells numbers of granules and globules accumulate, the 

 former being probably of a protein nature, and the latter of a fatty 

 composition. Gradually the alveoli become charged with a fluid 

 containing detached cells and fatty globules. The detached cells are 

 usually filled with granules, staining with osmic acid and seemingly 



FIG. 162. Section of mammary gland of woman. (From Sharpey 

 Schafer, after de Sinety.) 



a, Lobule of gland ; b, acini lined by cubical epithelium ; <, duct ; 

 #, connective tissue. 



identical with the colostrum corpuscles which have been observed to 

 occur in milk in the first few days after parturition, and occasionally 

 also at other times. These colostrum corpuscles have been seen to 

 exhibit amoeboid movements, and so are probably leucocytes which 

 have wandered into the lumina of the alveoli. 1 The secretory fluid 

 also contains cells which are supposed to have been detached from 

 the epithelium, but, as will be seen presently, there is some doubt 

 regarding this point. 



The alveoli secrete milk during lactation, not merely while 

 suckling is going on, but also at other times, so that milk tends to 

 collect in the ducts and especially in the reservoirs. It has been 



1 Sharpey Schafer, "The Mechanism of the Secretion of Milk," Text-Book 

 of Physiology, vol. i., Edinburgh, 1898. 



