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 (3) 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 



