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 Eauber, 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 view r s, 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 fiat 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. 
- Loc. cit. '•'■ Tub: Heidenhain, op. cit. 
