hunger, either by depressing the desire for food
or by stimulating appetite.
The drying of the membranes in the oral cavity
influences the sensation of thirst. Although thirst
may be due to a lack of water in body tissues, a
reduced salivary flow can produce a sensation of
SPEECH is controlled by the coordinated ac-
tion of several nerve functions. The speech center
is located deep in the brain, and from it nerve im-
pulses pass out to the larynx, which contains folds
of mucous membranes called vocal cords. When
air is forced from the lungs past these folds, cer-
tain sounds are produced, and, in conjunction
with the movements of the throat, lips, tongue,
and teeth, articulate speech results.
SLEEP is a period of unconsciousness when
the higher physical powers are quiet, although
body activities continue. It is usually considered
a period of rest in which constructive processes
build up and repair the body. Certain changes take
place during sleep: respiration is slowed; less
blood is sent to the brain and greater amounts to
to the extremities; digestion goes on, but at a
body temperature may drop
somewhat; and heart action is slowed.
THE ENDOCRINE SYSTEM
Homeostasis depends on the nervous and en-
docrine systems, since both are lines of com-
munication for body functions. The endocrine
system sends messages by chemical hormones that
are carried in the blood stream. These messages
aid in the control, development, and integration
of body functions.
The endocrine system is made up of glands of
internal secretion. These are called ductless glands
because they have no ducts to carry away their
secretions. The secretion of an endocrine gland
is called a hormone. It enters directly into the
blood or lymph circulation and eventually reaches
the gland, tissue, or organ it controls or in-
fluences. Very small quantities of hormones are
produced, since only a trace amount is needed to
produce the desired effect.
Most hormones can be extracted from the
glands of animals, and some can be produced syn-
thetically. Medical officers may prescribe these
isolated or synthetic hormones for patients who
are deficient in them or who might otherwise
Figure 3-51.The endocrine glands.
benefit from their use. The hormone-producing
glands include the pituitary, thyroid, parathy-
roids, adrenals, gonads, and pancreas (fig. 3-51).
The hypothalamus, a structure in the brain,
synthesizes chemicals that are secreted to the
pituitary gland to stimulate the release of its
The pituitary is a small, pea-sized gland
located at the base of the brain in the sella tur-
cica of the sphenoid bone. It is often called the
master gland of the body, because it influences
most other endocrine glands. It is divided into two
lobes, an anterior and a posterior. The anterior
lobe plays the more important role in influenc-
ing body functions. The hormones it produces
have a broad and significant range of effects.
SOMATOTROPIN, the growth hormone, in-
fluences body growth and development. During