THE MECHANISM OF THE SECRETION OF MILK. 
By E. A. S CHAFER. 
Contents : — General Considerations, p. 662 — Influence of the Nervous System, p. 663 
— Action of Pilocarpine and Atropine, p. 664 — Influence of Diet, p. 664 — 
Place of Formation of the Organic Constituents, p. 665 — Manner in which the 
Secreted Materials pass out of the Cells, p. 665 — Mechanism of the Discharge 
of Milk, p. 667. 
The composition of milk has been dealt with in a previous article 
(pp. 125 to 140). Here it may therefore he simply noted, with regard 
to its organic constituents, that these are remarkable in being peculiar 
to the milk, not occurring in any of the other secretions or tissues of the 
body (cf. however, footnote 1, p. 665), nor in foods which have not been 
prepared from milk. The mammary gland-cells, therefore, unquestional >ly 
form the products of secretion themselves from materials derived 
through the lymph from the blood, and cannot be regarded, except as 
concerns some of the inorganic substances, as acting merely as filtering 
agents for allowing the passage of materials in solution from the blood. 
And even with regard to the inorganic substances, 1 the proportion of 
these is so different from that in which they occur in the blood and 
lymph, that no nitration hypothesis appears in any way tenable even for 
these. The gland-cells are further peculiar in that they only, as a rule, 
function actively for a certain period after parturition, being at all 
other times entirely inactive, although capable occasionally — it is said 
even in the male — of being excited to activity by stimulation of the 
nipple by a sucking action, such as that performed by an infant. Prior 
to, and during such periods of activity, the whole gland becomes greatly 
enlarged, both by an increase in size of existing alveoli, and also, perhaps, 
by a sprouting out of new alveoli. The cells lining the alveoli become 
enlarged, and probably also multiply, for they are said to show evidence 
of karyokinesis. 
The alveolar cells begin to accumulate within them granules, 
partly of a proteid, partly of a fatty nature (although the latter may 
more fitly be described as globules), and the alveoli get filled, before 
there is any call for the pouring out of the secretion, with a clear fluid 
(coagulating to a finely granular material in pieces of the gland thrown 
into alcohol), which contains a few fatty globules of different sizes, and here 
and there cells filled with granules, staining with osmic acid, and appar- 
ently identical with the colostrum corpuscles which are found in the milk of 
1 Bunge has shown that, with the exception of iron, the inorganic substances of milk 
occur in nearly the same proportion as in the ash of new-born animals (•' Text-Book," Woold- 
ridge's translation, p. 107). 
