a. The endocrine glands are organs of internal secretion. Since they lack a
duct system, they are often referred to as ductless glands.
b. Since their secretions pass into the blood, they are usually well supplied with
The secretion of an endocrine organ is called a hormone. The hormone is a
chemical required in very small amounts for the proper development and/or functioning
of the body. (Note the similarity of this definition to that of a vitamin. However, the
hormone is produced within the body, and the vitamin is acquired from without.)
TARGET ORGAN AND FEEDBACK MECHANISM
When the hormone is secreted by the endocrine organ, it is carried by the blood
to the appropriate organ, the target organ. In addition, the level of activity of the target
organ often affects the activity of the endocrine organ. Thus, there is a feedback
mechanism that causes the endocrine organ to secrete just the right amount of
Section II. THE PITUITARY BODY
a. The pituitary body is located immediately under the brain. It is in a special
hollow of the floor of the cranial cavity. The pituitary body is actually two glands: the
posterior pituitary gland and the anterior pituitary gland.
b. As a whole, the pituitary body produces a large number of hormones. These
affect many tissues of the body. Many of these hormones are referred to as tropins (or
trophins) because they cause development or activity of the tissues.
POSTERIOR PITUITARY GLAND
In the embryo, the posterior pituitary gland develops as an outcropping
(hypophysis) of the inferior part of the brain. Later in life, the posterior pituitary gland
remains connected to the forebrain-stem. There is a series of nuclei in the fore-
brainstem which are together referred to as the hypothalamus. The hormones of the
posterior pituitary gland are actually produced in the hypothalamus. The hormones
pass from the hypothalamus to the posterior pituitary gland by way of neurosecretory
fibers. From the posterior pituitary gland, the hormones are secreted into the blood.
The main hormones of the posterior pituitary gland are: