8 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



manipulation of the teats in milking causes the hyper- 

 asmia and increased secretion set up by stroking the teats 

 to continue until the secreting cells are exhausted, when 

 the flow of milk ceases and the udder becomes smaller 

 in size and relaxed. The manipulation of one teat pro- 

 duces hypersemia and stimulates secretion in all four 

 quarters of the udder. The second phase of milk secre- 

 tion is therefore due to the mechanical stimulation result- 

 ing from the manipulation of the teats in milking. The 

 stimulating apparatus is in the teats, but it is not clear 

 how the stimuli .are transmitted. The udder is richly 

 supplied with nerves from the sympathetic system and 

 from the lumbar plexus, and yet secretion can go on 

 after the greater part of the nerve apparatus is discon- 

 nected. 



Three factors are concerned in the second phase of 

 milk secretion: (1) The vasodilator nerves are stimu- 

 lated reflexly by the mechanical manipulation of the 

 teats in milking, producing a hypersemia and bringing 

 a rich supply of milk-forming material to the secreting 

 cells. (2) The operation of milking stimulates also the 

 secretory and the motor nerves ; stimulation of the secre- 

 tory nerves accelerating the secretory processes in the 

 cells of the alveoli, and stimulation of the motor nerves 

 causing a contraction of the alveoli and ducts and forcing 

 the milk into the cistern. (3) The " milk-formers," which 

 circulate in the blood, stimulate the secretory cells 

 (Zietzschmann). 



Under certain abnormal conditions the secretion of 

 the second phase stops before the usual amount of milk 

 is obtained. This may result from fright, sudden anx- 

 iety, and in sensitive animals from unusual manipulation 

 of the teats (strange milkers) , from sore teats, etc. The 



