236 Neuroanatomy: Draw It to Know It
The Ear
Here, we will draw a coronal view of the ear with the
subject facing towards us. Begin with our planes of
orientation. First, draw the medial–lateral plane; then,
the anterior–posterior plane; and fi nally, the superior–
inferior plane. Now, draw the outer ear. Th en, draw the
external ear canal, which extends through the tympanic
portion of the temporal bone, just in front of the mas-
toid process. Next, draw the tympanic membrane.
Indicate that sound waves pass through the external ear
canal to vibrate the tympanic membrane.
Now, draw the middle ear canal, which also lies mostly
within the tympanic portion of the temporal bone.
Indicate that it contains three ossicles — from lateral to
medial, the malleus, incus, and stapes, which are the
Latin terms for their shapes: “hammer,” “anvil,” and “stir-
rup,” respectively. Next, indicate that the stapes abuts the
oval window. When sound is transmitted through the
ossicles, the stapes pushes the oval window into the inner
ear (drawn soon). Now, show that the eustachian tube
extends from the middle ear into the nasopharynx; it
causes your middle ears to equilibrate with the atmo-
spheric pressure in your nasopharynx when you swallow.
Two important muscles exist within the middle ear
canal: the tensor tympani, which is innervated by the
trigeminal nerve and which acts on the tympanic mem-
brane, and the stapedius muscle, which is innervated by
the facial nerve and which acts on the stapes.
Next, we will draw the inner ear canal, which lies
within the petrous portion of the temporal bone. Divide
the inner ear into three diff erent parts. First, draw the
semicircular canals, which lie in superior-lateral position
and serve vestibular function; then, draw the cochlea,
which is shaped like a snail’s shell, and which lies in ante-
rior-inferior position and serves auditory function; and
fi nally, draw the vestibule, which lies in between them
and is involved in both auditory and vestibular func-
tions — it transmits sound waves from the oval window
to the cochlea and contains the otolith organs, which
provide vestibular cues.^1
Now, let’s draw the three ducts that form within the
cochlea. First, draw the cochlear duct (scala media);
then, label the vestibular duct (scala vestibuli), which is
continuous with the vestibule; and fi nally, label the tym-
panic duct (scala tympani), which ends in the round
window (aka the secondary tympanic membrane). Show
that the vestibular and tympanic ducts connect at the
apex of the cochlea (aka the helicotrema). Reissner’s
membrane separates the vestibular duct from the
cochlear duct and the basilar membrane separates the
cochlear duct from the tympanic duct. Th e vestibular
and tympanic ducts are fi lled with perilymphatic fl uid,
whereas the cochlear duct is fi lled with endolymphatic
fl uid. Perilymphatic fl uid is like extracellular fl uid in that
it is high in sodium and low in potassium, whereas endo-
lymphatic fl uid is like intracellular fl uid in that it is high
in potassium and low in sodium. Patients with Ménière’s
syndrome, which causes bouts of vertigo, low-frequency
hearing loss, and ear fullness, are commonly treated with
salt-wasting diuretic medications because the syndrome is
thought to be due to pathologically elevated endolym-
phatic sodium concentration. In more severe cases of
Ménière’s syndrome, vestibular ablation is performed and,
also, aminoglycosides (specifi cally gentamicin and strep-
tomycin) are used to destroy the vestibular end-organ
with relative preservation of the auditory cells.
Now, indicate that when the oval window vibrates, a
fl uid wave passes through the vestibule, which then
passes through the vestibular duct, across the apex of the
cochlea into the tympanic duct, and then through the
tympanic duct to push the round window into the air-
fi lled middle ear canal. In this process, the auditory sen-
sory organ, the organ of Corti, which lies along the
basilar membrane, is activated for sound detection.
High-frequency sounds activate hair cells at the base of
the cochlea (near the oval and round windows) whereas
low-frequency sounds activate hair cells at the apex of
the cochlea. Th e basilar membrane is thinnest at its base
and widest at its apex.