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 ifi 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. 



