SEPTEMBER 1955]     S. N. LUKTUKE                                            215

         (v) Adrenals. These lie near the kidney and produce adrenaline and cortical
         hormones.

         (vi) Placenta. This acts as an endocrine gland during pregnancy.

As already mentioned, the heifer in calf, for the first time, has a very
little development of the alveoli in her udder up to the fourth month of gestation.
After this period the level of oestrogens in the blood system rises, the alveolar develop-
ment begins and this progresses until the end of pregnancy. Progesterone, the
hormone of the corpus luteum, is responsible for the development of the duct system
in the udder. Oestrogen and progesterone, in conjunction with two hormones from
the anterior portion of the hypophysis (mammogenic duct growth hormone and
mammogenic lobule-alveolar growth hormone), are concerned with the growth of the
udder and if there is a short supply of any one of these hormones, full development of
the mammary gland does not take place. At the time of birth, the level of oestrogen
in relation to progesterone is very high and this gives rise to the onset of parturition.
At this time, a hormone is liberated from the posterior part of the pituitary. This
is called as oxytocin. This hormone assists in the contraction of walls of the uterus
at the time of parturition and helps the expulsion of foetus from the uterus. During
advanced pregnancy, a hormonal imbalance is created because of the excess of
oestrogenic hormones secreted by the placenta, resulting in the arrest of milk pro-
duction. Placenta thus checks the milk secretion and at parturition this check is
removed. When the young one is born, the pituitary gland is called upon to produce
a hormone prolactin, which causes the secretion of milk in the developed udder.
Oxytocin controls the ejection of milk from teats. Suckling of teats by calf sets
up certain stimuli which influence the liberation of oxytocin through the central
nervous system, and increases the glandular pressure to squeeze out the milk from
alveoli and smaller ducts. Similar stimuli are produced by rattling of milk buckets
in a milking shed, washing the udder with warm water, etc. Stimuli of a different
nature such as fright cause the secretion of adrenaline which prevents the ejection
of milk.

A constant production of the lactogenic hormone—prolactin—by the pituitary
is necessary for continued milk secretion. If the pituitary is removed from a
lactating animal, the milk ceases to be formed.

Proper functioning of the thyroid gland is necessary for normal milk secretion.
Surgical removal of thyroid has an adverse effect on lactation. Cortical hormones of
the adrenal gland also influence lactation. Removal of the adrenals rapidly stops
milk secretion.

INCREASING ESTABLISHED LACTATIONS BY HORMONAL METHODS

Anterior pituitary hormones, if administered to a cow during the declining
phase of her lactation, cause an increase in the milk yield. Another hormone.
thyroxine, is also found to give similar results. A preparation called iodocasein
obtained by treating milk proteins with iodine can give similar response as that by
thyroxine. Recently in the U. K., a large experiment was undertaken involving