668 MECHANISM OF THE SECRETION OF MILK 
The discharge of milk. — The discharge of milk from the ducts, 
which is produced by the action of sucking or milking, is partly the 
result of direct mechanical pressure upon the gland, and especially 
upon the milk reservoirs of the larger ducts, partly due to a con- 
traction of the plain muscular tissue which accompanies these ducts, 
and which appears to be set in action by mechanical stimulation 
of the nipple. The plain muscular tissue occurs also in the nipple 
itself in some abundance, and by its contraction causes a kind of 
erection and increased prominence of the nipple. This probably serves 
the purpose of keeping open the mouths of the gland-ducts which open 
upon the rounded apex of the nipple, thus allowing of the free outflow 
of the secretion. 1 The flow is also in all probability assisted by a vis 
a tergo, derived from the swelling of the whole gland by the reflex 
dilatation of its arterioles, and consequent increase of capillary pressure, 
and of lymph exudation ; and, to a slight degree also, by newly secreted 
milk, which begins to be formed by the gland-cells, in response either 
to this increased supply of blood and lymph, or to reflex secretory 
influences passing directly to the gland-cells. 2 
tion of the nuclear substance into fat. The latter could find no mitoses during lacta- 
tion, although he found two or three nuclei in each cell. He also describes the accumula- 
tion in the cells of albuminous granules which undergo peculiar changes of form, and 
are ultimately extruded into the alveoli and there dissolved. These observations require 
corroboration. 
1 Cf. Hellier "On Nipple Reflex," Brit. Med. Journ., London, Nov. 7, 1896. 
2 Cf., however, what has been already said on this subject on p. 663. 
