94 



The Endocrine Organs 



If after a first full dose of pituitary extract has been given, a second 

 dose succeed it at a short interval, no further flow of secretion from the 

 exhausted mamma can be produced. 1 The effect of the autacoid, therefore, 

 is not — at any rate immediately— to cause the cells to form and secrete 

 milk, but only to cause the alveoli to empty themselves of the milk which 

 has been previously formed and secreted within them. The simplest 

 manner in which one can conceive this to occur is by contraction of (plain) 

 muscular tissue around the alveoli. In support of this conception, I have 

 ~v succeeded in detecting in the 



*. walls of the mammary alveoli 



T-^r long, rod-shaped nuclei im- 



mediately external to the 

 epithelium. These nuclei 

 exactly resemble those of in- 

 voluntary muscle-cells, and 

 probably belong to a thin 

 muscular layer which is situ- 

 ated, like the muscular tissue 

 of the sweat glands, between 

 the basement membrane and 

 the epithelium of the alveoli. 

 The action of pituitary ex- 

 tract upon milk secretion 

 differs therefore from its 

 effect upon the secretion of 

 urine in the fact that in the 

 last-named case there is an 

 actual stimulation of the 

 renal cells to abstract fluid 

 from the blood, whilst in the 

 mammary gland there occurs 

 merely a contraction of the walls of the alveoli and discharge of fluid 

 previously secreted. 



Apart from the pouring out of the contents of the alveoli — which, when 

 the gland is intact, shows itself as a tendency of the alveoli to empty 

 themselves towards the nipple, — the pituitary galactagogue has little 

 or no effect on the total production of milk. This at least is the result 

 which was obtained in the goat by Findlay, and in cows by Gavin. Later 

 observers (Hammond, Sutherland Simpson) have obtained a slight increase 

 in the diurnal yield of both goats and cows ; and also a slightly increased 

 amount of fat in the milk produced under the influence of the autacoid. 



Mackenzie found (in the cat) that atropine does not arrest the flow of 

 milk obtained as the result of pituitary injection (an observation which has 



1 This is not an instance of tachyphylaxis, but is due to the fact that there is now no 

 milk left in the alveoli. 



Fig. 65. — Section including the adjacent parts of two 

 mammary glands of a lactating cat. One of them had 

 discharged the whole of its milk as the result of an 

 intravenous injection of pituitary extract, the nipple 

 having been excised to allow the milk to flow out freely. 

 In the other the alveoli are distended with milk. Low 

 power. 



