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 been 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, (a) 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-secreting 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. Nor 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 Loc. cit. 



- The microscopical changes in the cells of the mammary gland during secretion have 

 been 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- 



