and functions that do so make up the vestibular
system. The bony labyrinth, which also houses the
cochlea, supports the membranous labyrinth. Five
fluid-filled structures within the membranous
labyrinth sense motion and movement: the three
semicircular canals, which sense rotational move-
ment, and the saccule and utricle, which sense
linear movement. The saccule senses movement
that is up-and-down; the utricle senses back-and-
forth and left-to-right movement. These structures
are all open to one another; however, they form a
closed network among themselves that contains
endolymph. Movement causes shifts in pressure of
the endolymph, which nerve cells register and
send as electrical impulses to the vestibular nerve.
The vestibular nerve conveys these impulses to
the eighth cranial nerve, which in turn carries
them to the brain. The brain interprets the
vestibular messages along with other input from
sensory nerve cells (proprioceptors) located
throughout the body, nearly instantaneously
responding with neuromuscular signals that initi-
ate movement.
Extremes of movement, such as rapid swinging
or spinning, can temporarily disrupt the vestibular
system, causing dizziness and NAUSEA. Some people
experience such symptoms with less extreme
movement, such as riding in a car, boat, or airplane,
known commonly as motion sickness. More seri-
ous dysfunctions and disorders of the vestibular
system, such as MÉNIÈRE’S DISEASEand LABYRINTHITIS,
can result in debilitating loss of balance.
Functions of the nose: breathing and smell The
nose protrudes from the front of the face. Its
placement allows it to draw air into the body in
one of life’s most basic activities, BREATHING. Most
of the nose’s external structure is CARTILAGEand
SKIN; the nasal bone is less than one half inch long.
The ethmoid, vomer, and maxilla bones frame the
back of the nose. Ridges in these bones, the nasal
conchae, direct the flow of air into the SINUSES.
These chambers, along with the nose’s mucus lin-
ing, moisturize and warm each breath so it does
not irritate the airways and lungs. A tissue wall,
the nasal septum, divides the nasal cavity into two
channels, the nostrils. Tiny hairs line the inner
nostrils and are responsible for keeping the nasal
passages clear of debris.
The nose is also the body’s organ of smell,
responsible for the functions of olfaction. The first
cranial nerve (olfactory nerve) terminates in the
olfactory bulb and a bristlelike patch of olfactory
nerve endings along the roof of the nose. These
olfactory nerve endings detect the presence of
odor molecules in the air that enters the nose.
Fibers of the palatine nerves, which detect taste,
are also present along the floor of the nose,
though not nearly in the abundance with which
they infiltrate the mouth. The brain interprets the
blend of nerve impulses from the palatine nerve
endings and the olfactory nerve endings and inte-
grates them into perceptions of flavor.
Functions of the mouth and throat: taste, swal-
lowing, and speech The mouth and throat make it
possible to eat and speak. The powerful masseter
muscles open and close the mandible (lower jaw),
generating over 500 pounds per square inch of
pressure as the TEETHcome together to bite and in
excess of 3,500 pounds per square inch of force at
the back teeth (molars) with chewing. The hyoid
bone helps anchor the back of the tongue, another
powerful MUSCLE. The SALIVARY GLANDS, present in
pairs on each side of the mouth, produce two to
three pints of saliva every day. This watery liquid
contains enzymes and mixes with food to begin
breaking it down, an early stage of digestion, as
well as to soften it for swallowing. The cheeks,
tongue, and lips help contain food within the
mouth and push it to the back of the throat for
swallowing; they also shape the flow of air and
create the formation of words during speech.
These functions require muscular control and
coordination.
The sense of taste is called gustation. Though
common perception is that the bumps on the
tongue are the taste buds, taste buds are micro-
scopic. The bumps are called papillae; they contain
clusters of taste buds. Each taste bud contains
dozens of taste cells. Though taste buds for the
four categories of taste—sweet, sour, salt, and bit-
ter—are present throughout the mouth, the
roughly 10,000 of them on the tongue align in
patterns of concentration:- Taste buds on the tip of the tongue are concen-
trated to detect sweet.
4 The Ear, Nose, Mouth and Throat