PHYSIOLOGY OF MILK SECRETION 7 
and consequently the milk cistern remains practically 
empty. 
The second phase of milk secretion begins when the 
udder is stimulated reflexly by manipulation of the teats. 
The udder becomes fuller and more tense, the gland sub- 
stance firmer. The teats lengthen and become rigid 
and divergent, while the wrinkles disappear from the 
skin covering them. The udder is distended and the 
cisterns are full of milk. The milk has been “let down.” 
All these changes are brought about by the downward 
stroking of the teats, which causes reflexly (1) dilation 
of the blood vessels and an increased flow of blood to 
the udder, (2) contraction of the walls of the alveoli 
and ducts, which forces the milk down into the cisterns, 
and (8) increased secretion of milk. The same effect 
may be produced reflexly by an irritation of the inter- 
nal genital organs such as may result from irrigation 
of the uterus or vagina, or manual removal of the pla- 
centa, and also by psychic influences like the sight of 
the calf, the clatter of the milk vessels, the sound of milk 
drawn from another cow into a pail, etc. When the 
interval between milkings is too long the milk is “let 
down” spontaneously, but in this case it occurs grad- 
ually. 
As soon as the milk is sufficiently “let down” the 
withdrawal of milk may be begun. The descending pres- 
sure exerted by the hand upon the contents of the milk 
cistern in the operation of milking opens the sphincter 
between the teat canal and the cistern, permitting the 
milk to escape. This pressure operates perpendicularly 
to the wall of the cistern, and when the bottom of the 
cistern is pointed or conical the sphincter is opened more 
readily than when the bottom is flat (see Fig. 4). The 
