PHYSIOLOGY OF MILK SECRETION 9 
distended udder is suddenly reduced in size and becomes 
relaxed. This is due to a reflex contraction of the blood 
vessels, which reduces the blood supply, and to the empty- 
ing of the milk cisterns. Contraction of the blood-vessels 
slightly enlarges the alveoli and ducts, creating a va- 
cuum, and the milk in the cisterns is drawn up into the 
alveoli and ducts. It is then said the cow “draws the 
milk up,” or “will not give down.” In these cases the 
physiological effect of the manipulation of the teats in 
milking is overcome by a stronger impulse. If these 
impulses are repeated frequently, or are due to more 
or less permanent conditions, like chronic sores on the 
teats, the shortening of the second phase may become 
habitual or permanent. 
The secretion of the first phase is passive and slow. 
According to Zietzschmann, it is due to the stimulant 
effect upon the gland cells of the “milk-formers” cir- 
culating in the blood. 
The secretion of the second phase is active and rapid, 
and is instituted and continued by the manipulation of 
the teats in milking. 
Within certain limits, increasing the number of milk- 
ings, thus shortening the period between milkings, will 
increase the total daily yield of milk. If milked three 
times in twenty-four hours, a cow will usually yield a 
greater total quantity of milk per day than when milked 
twice in twenty-four hours. 
The composition of the milk is different in the first 
and second phases. The first milk drawn from the udder 
at any milking will contain a smaller proportion of 
solids, especially fat, than the succeeding milk. This is 
not due to the fat rising to the top of the fluid in the 
udder, nor to the adhesion of the fat globules to the walls 
