666 MECHANISM OF THE SECRETION OF MILK. 



constituents, which are probably, as already stated, the proteids and carbo- 

 hydrates derived from the food. 



As to the manner in which the secreted materials of the milk 

 pass out of the secretory cells. If we put aside, as resting upon no 

 solid basis of fact, the suggestion of Strieker, which was taken up. more 

 seriously by Kauber, 1 that the organic materials of the milk are carried 

 into the alveoli by emigrated leucocytes, which there break down and set 

 free their proteid, fatty, and other constituents, we find ourselves face 

 to face with three possible methods by which the secreted materials 

 which are formed and accumulate within the gland-cells may pass into 

 the lumen of the alveoli. The three methods are as follows : 



1. The cells may, as in the case of the sebaceous glands, bodily 

 break loose, and, becoming detached and disintegrated, set free their 

 contents within the alveoli. 



2. A part only of each cell, namely, the free end, may break loose, 

 become detached, and disintegrate. 



3. The cells may extrude their secreted materials into the alveoli,much 

 as in the case of other secretions, without undergoing any histological 

 disintegration. 



Of these three views, the first has found support mainly on the ground 

 of the analogy with what happens in the case of the sebaceous glands of the 

 skin (with which the mammary glands might be looked upon as in a certain 

 sense homologous), in which such a complete disintegration of the whole 

 cell occurs, its place being supplied by another cell, which is produced by 

 cell-division. Moreover, the colostrum corpuscles have been regarded as 

 examples of such detached cells filled with secretion, which have not 

 become disintegrated. Those corpuscles, however, as we have seen, are 

 rather to be looked upon as of the nature of leucocytes than as epithelial 

 cells ; nor do we find such evidence of cell multiplication in the mammary 

 gland as would be at all sufficient to account for the very large number 

 of cells which would have to become detached in order to furnish the 

 organic constituents of the secretion. Heidenhain 2 has calculated that 

 the gland-cells would have to be totally renewed five times in the 

 course of every twenty-four hours, in order to yield the solids of the 

 milk. The second view may be looked upon as, in a sense, a modifica- 

 tion of the first one. It was due originally to Langer, and has been 

 ably advocated by Partsch and Heidenhain. 3 According to this 

 view, the secretion products which are formed in the gland become 

 gradually accumulated within the free ends of the cells, which in the 

 meanwhile lengthen out, and in place of being flat or cubical become 

 columnar and project into the lumen of the alveolus. The enlarged free 

 end is then supposed to burst or to become detached and disintegrated, 

 and thus to set free the accumulated products, while the fixed ends of 

 the cells (with the nuclei) are supposed to remain, ready to again go 

 through a similar process. 



The evidence adduced in favour of this view is chiefly of a histological 

 nature. It is the case that in some alveoli of glands in full secretion, 

 the cells are occasionally seen projecting somewhat prominently into 

 the lumen ; and it is certainly the case also that the cells, and perhaps 

 especially such prominent parts, contain fatty globules, similar in 



1 " Ueber den Ursprung der Milch," Leipzig, 1879. 



2 Loc. cit. 3 Vide Heidenhain, op. cit. 



