ix THE SKIN AND CUTANEOUS GLANDS 509 



with the secretion of the sebaceous glands, by the disintegration 

 and liquefaction of the older cells, in proportion as new cells 

 replace those destroyed and converted into milk. This view rests 

 on the common origin and homology of the two kinds of glands. 

 It also finds support in the theory that prevailed as to the origin 

 of the colostrum corpuscles found in the first milk, which is secreted 

 in the last days of pregnancy and the first 3-4 days after parturi- 

 tion. The colostrum corpuscles are true cells, mostly nucleated, 

 round, and very granular. By some they have been regarded as 

 modified gland cells, which become detached and pass into the 

 secretion previous to complete disintegration, which they undergo 

 later, when colostrum corpuscles are no longer present in the milk. 

 As soon as the mammary glands are excited to activity, the 

 alveolar cells fill with granules and globules of fat, and the 

 alveolar spaces with a clear fluid in which fat globules and 

 occasional colostrum corpuscles are seen to float. 



The colostrum corpuscles, however, have not the appearance of 

 epithelial cells, and on the warm stage of the microscope they 

 exhibit amoeboid movements similar to those shown by leucocytes. 

 They are therefore supposed by many to be leucocytes that have 

 wandered from the lymph sinuses of the perialveolar tissue into 

 the lumen of the alveoli, and not cast-off epithelial cells. 



The theory that milk is formed by regeneration of the epithelial 

 cells is discounted by the fact that no such rapid and persistent 

 cell-multiplication can be detected in the mammary gland as 

 would account for the organic constituents of the milk, on the 

 theory of detritus from disintegrated cells. Heidenhain, indeed, 

 calculated that the gland-cells would have to be renewed at least 

 five times in every 24 hours, if they are to provide the solid 

 materials of the milk. 



Partsch (1880) advocated another view, which was accepted 

 by Heidenhain. According to this, the secretion products formed 

 in the gland become gradually accumulated at the free ends of 

 the cells, which lengthen out and become columnar, projecting 

 unequally into the lumen of the alveolus. The enlarged free end is 

 supposed to become detached, and discharge the accumulated 

 products, while the outer part of the cell (with the nuclei) remains 

 intact, and repeats the same process. 



In support of this hypothesis they adduce certain histological 

 researches carried out on the mammary gland of the bitch in 

 different phases of secretory activity (Fig. 140). At the commence- 

 ment of lactation the alveoli were distended by a clear secretion, 

 in which fat globules and a few colostrum corpuscles floated. 

 The epithelium was flattened against the basement membrane, and 

 contained fatty globules of various sizes. During full secretory 

 activity, on the other hand, i.e. when lactation was fully developed, 

 the cells became first cubical and then columnar, and vary in size ; 



