8 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
manipulation of the teats in milking causes the hyper- 
zmia 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 hyperemia 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 hyperemia 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 
