Page 630 - Fundamentals of anatomy physiology

The External Ear
The external ear includes the outer fleshy and cartilaginous
auricle, or pinna, which surrounds a passageway called the
external acoustic meatus, or auditory canal. The auricle pro-
tects the opening of the canal and provides directional sensitiv-
ity. Sounds coming from behind the head are blocked by the
auricle, but sounds coming from the side or front are collected
and channeled into the external acoustic meatus. (When you
“cup” your ear with your hand to hear a faint sound more
clearly, you are exaggerating this effect.) The external acoustic
meatus ends at the tympanic membrane (tympanon, drum) or
eardrum. This thin, semitransparent sheet separates the external
ear from the middle ear.
The tympanic membrane is very delicate. The auricle and
the narrow external acoustic meatus provide some protection
from accidental injury. In addition, ceruminous glands—
integumentary glands along the external acoustic meatus—
secrete a waxy material that helps keep out foreign objects or
small insects. The canal is also lined with many small, out-
wardly projecting hairs. These hairs trap debris and also provide
increased tactile sensitivity through their root hair plexuses.
The slightly waxy secretion of the ceruminous glands, called
cerumen, also slows the growth of microorganisms and re-
duces the chances of infection.
The Middle Ear
The middle ear, or tympanic cavity, is an air-filled chamber
separated from the external acoustic meatus by the tympanic
membrane. The middle ear communicates both with the na-
sopharynx (the superior portion of the pharynx), through the
auditory tube, and with the mastoid air cells, through a num-
ber of small connections (Figures 17–21 and 17–22). The audi-
tory tube is also called the pharyngotympanic tube or the Eusta-
chian tube. About 4 cm (1.6 in.) long, it consists of two portions.
The portion near the connection to the middle ear is narrow
and is supported by elastic cartilage. The portion near the
opening into the nasopharynx is broad and funnel shaped. The
auditory tube equalizes pressure on either side of the tympanic
membrane. Unfortunately, the auditory tube can also allow mi-
croorganisms to travel from the nasopharynx into the middle
ear. Invasion by microorganisms can lead to an unpleasant
middle ear infection known as otitis media.
The Auditory Ossicles. The middle ear contains three tiny ear
bones, collectively called auditory ossicles. These ear bones
connect the tympanic membrane with one of the receptor com-
plexes of the internal ear (Figures 17–21 and 17–22). The three
auditory ossicles are the malleus, the incus, and the stapes.
The malleus (malleus, hammer) attaches at three points to the
interior surface of the tympanic membrane. The incus (incus,
anvil), the middle ossicle, attaches the malleus to the stapes
(stapes, stirrup), the inner ossicle. The edges of the base of the
Chapter 17 The Special Senses 617
stapes are bound to the edges of the oval window, an opening in
the temporal bone that surrounds the internal ear.
The articulations between the auditory ossicles are the
smallest synovial joints in the body. Each has a tiny joint cap-
sule and supporting extracapsular ligaments.
Vibration of the tympanic membrane converts arriving
sound waves into mechanical movements. The auditory os-
sicles act as levers that conduct those vibrations to the internal
ear. The ossicles are connected in such a way that an in–out
movement of the tympanic membrane produces a rocking
motion of the stapes. In this way, the ossicles function as a
lever system that collects the force applied to the tympanic
membrane and focuses it on the oval window. Considerable
amplification occurs, so we can hear very faint sounds, because
the tympanic membrane is 22 times larger and heavier than
the oval window.
That degree of amplification can be a problem when we
are exposed to very loud noises. In the middle ear, two small
muscles protect the tympanic membrane and ossicles from
violent movements under very noisy conditions:
17 1. The tensor tympani (TEN-sor tim-PAN-e. ) muscle is a
short ribbon of muscle originating on the petrous part of
the temporal bone and the auditory tube, and inserting on
the “handle” of the malleus. When the tensor tympani con-
tracts, it pulls the malleus medially, stiffening the tympanic
membrane. This increased stiffness reduces the amount
of movement possible. The tensor tympani muscle is in-
nervated by motor fibers of the mandibular branch of the
trigeminal nerve (V).
2. The stapedius (sta-PE. -de. -us) muscle, innervated by the
facial nerve (VII), originates from the posterior wall of the
middle ear and inserts on the stapes. Contraction of the sta-
pedius pulls the stapes, reducing movement of the stapes at
the oval window.
The Internal Ear
Receptors in the internal ear provide us with the senses of
equilibrium and hearing (Figures 17–21 and 17–23).
The superficial contours of the internal ear are formed by a
layer of dense bone known as the bony labyrinth (labyrinthos,
network of canals). The walls of the bony labyrinth are con-
tinuous with the surrounding temporal bone. The inner con-
tours of the bony labyrinth closely follow the contours of the
membranous labyrinth, a delicate, interconnected network
of fluid-filled tubes. The receptors of the internal ear are found
within those tubes.
Between the bony and membranous labyrinths flows
perilymph (PER-i-limf), a liquid that closely resembles cerebro-
spinal fluid. The membranous labyrinth contains endolymph
(EN-do. -limf), a fluid with electrolyte concentrations that differ
from those of typical body fluids. The physical relationships