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 



