THE DISCHARGE OF MILK. 667 
appearance to those of the milk. Nevertheless, it must be admitted 
that such appearances, although they may occasionally be seen, are 
decidedly rare. It is so much more common to see, even in the most 
actively secreting glands, the alveolar cells uniformly flattened, or at most 
very slightly projecting. that I should not hesitate to say that the columnar 
appearance, which has heen described and figured by Heidenhain, is 
quite exceptional, and is in all probability due to the alveoli in which 
it occurs being collapsed. Every histologist is aware of the extreme 
differences in shape which are produced in epithelial cells by altera- 
tions in the conditions of the surface which they cover. Thus, even 
the extremely flattened epithelial cells which line the blood vessels 
may, when examined in sections of vessels which have been hardened in 
a contracted and collapsed condition, project like columnar epithelium 
cells into the lumen of the vessel. And the differences in height of 
the cells lining the alveoli of the mammary glands may very well be 
similarly produced. This is indeed rendered probable from the observa- 
tion of Heidenhain, 1 that in different lobules of a gland the cells vary 
in height, but in the same lobule they have the same height; and by 
the additional observation of the same observer, («) that in a bitch 
which was suckling seven vigorous puppies the cells were very high ; 
(b) that in another well-fed milch bitch, which was not sucked for 
forty-eight hours, they were remarkably low ; for the alveoli in these 
cases would be flaccid and tense respectively. 
The histological evidence in favour of this view must therefore be 
admitted to be extremely weak, nor, except perhaps in the case of the 
unicellular glands of some invertebrates, and the similar unicellular 
secreting structures which form the mucus-secretino; goblet-cells of verte- 
brates, is there any analogous instance of the extrusion of a secretion 
by the breaking down of part of the gland-cells. Moreover, the 
argument which was used by Heidenhain against the first view, that 
it would involve the renewal of the substance of the epithelial cells of 
the mammary gland five times in twenty-four hours, will apply with 
slight modification equally to the second, and adds a further consider- 
able difficulty to its acceptance. 
The third view, on the other hand, has the analogy of nearly all 
the other secretory structures to support it. It involves no necessity 
for assuming such an enormous building up and breaking down of 
protoplasm as is required for the other two ; and although we must 
admit that the present state of our knowledge does not permit us 
to understand how and why it is that certain substances are formed 
in these cells, and pass from them into the lumen of the alveolus, the 
same admission must be made for all other secretions. Xor is the fact 
that the fat of the milk is extruded from the cells in an undissolved 
condition any obstacle to the acceptance of the view in question, since 
it is probable that the granules which are found in many other secretory 
cells {e.g. those of the salivary glands), and which are passed into the 
lumen of the alveolus, are extruded as granules, and are first dissolved 
in the secretion outside the cells. 2 
1 Lot. cit. 
- The microscopical changes in the cells of the mammary gland during secretion have 
heen recently made the subject of study by Steinhaus (Arch. f. Physiol., Leipzig, 1892, 
Suppl., S. 54) and Szabo {ibid., 1896, S. 32). The former finds evidence of frequent mitotic 
division of the cell nuclei (without subsequent division of the cells), and of transforma- 
