into the blood stream, while exocrine use a duct or net-
work into the body. Glands important to the human
body are the hypothalamus, which secretes hormones
to regulate the pituitary gland; the pineal gland, which
secretes melatonin, a hormone that deals with daily
biological rhythms; the pituitary gland, which secretes
hormones that influence other glands and organs that
deal with growth and reproduction; the thyroid gland,
which regulates metabolism and blood calcium levels;
the parathyroid gland, which regulates the use of calci-
umand phosphorus; the thymus gland, which stimu-
lates the immune system’s T cell development; the
adrenal gland, which secretes the male hormone,
androgens, and aldosterone, which helps maintain the
body’s salt and potassium balances, and epinephrine
(adrenaline) and norepinepherine (noradrenaline); the
pancreas gland, which secretes insulin, which controls
the use of sugar in the body, and other hormones
involved with sugar metabolism; the ovaries, which
secrete female hormones such as estrogen, which main-
tains female traits, and progesterone for pregnancy; the
eccrine gland, whose excretory canal emerges directly
onto the skin’s surface; the exocrine gland, which
secretes products that get directly eliminated, at the
level of the skin, or through a mucous membrane; and
the holocrine gland, whose secretions result from the
destruction of the cells forming it.
Aplant gland is usually a bump, depression, or
appendage on the surface or within that produces a
sticky or greasy viscous fluid such as oil or resin. The
floral nectary is a gland in the flower that secretes a
sugary fluid that pollinators utilize for food, while the
extrafloral nectary is a gland on the nonflower part
that secretes a sugary fluid that serves the same pur-
pose. A gland-dot is a tiny pore that secretes fluid;
glandular hairs bear glands. Oil glands can often be
seen on a plant’s areole, and irregular (not round) leaf
oil glands can be either an island oil gland that is not
connected to veinlets or an intersectional oil gland that
is connected to veinlets.glaucoma A disease of the eye characterized by loss
of vision due to an increase in the pressure of fluid
within the eye (intraocular pressure) that leads to dam-
age to the optic nerve and loss of vision in both eyes.
Can eventually lead to blindness. The disease affects
about 6 million people worldwide. In the United States,
about 3 million people are affected.glial cell Nonimpulse-conducting cells that make up
half the weight of the brain; in the central nervous sys-
tem they are 10 times as numerous as neurons and act
as support cells by forming insulation around the neu-
rons to protect them. Their support functions provide
myelin for axons, and they act as housekeepers after
cell damage or death by cleaning up. Glial cells also
play an important role in the early and continuing
development of the brain. A small hormonelike protein,glial cell 147An example of alpine conditions and a glacier at full summer
retreat. (Glaciers expand and contract with the season.)
(Courtesy of Tim McCabe)