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The Normal Ear
What is the anatomy of the normal ear?
The uniquely shaped outer ear is easy for all to see. Some ears stick out; some are close to the head. Each has its own distinct shape. Most 18-month-old children can point to an ear if asked. The shape of the outer ear focuses on the small hole that leads to a dark canal that is a mystery to most parents. Beyond what we know from "Q-Tip exploration" lies a space as unfamiliar as a forbidden temple hidden in a jungle.
The ear canal is a gently curved, skin-lined tube that travels through the bones of the skull. It deadends into the eardrum, or tympanic membrane. The eardrum is a thin, semitransparent (or translucent) membrane that separates the external ear canal from the middle ear space. Sound waves in the environment cause the eardrum to vibrate. The middle ear is an air-filled cave lying just beyond the eardrum. It is the structure that transmits and amplifies sound from the eardrum to the inner ear.
The middle ear is one of the few structures in the body that is the same size at birth as it is in adulthood. This cavity is a little more than one-half-inch tall and between one-quarter-and one-eighth-inch wide a tall, narrow, air-filled box. The outer wall is the flexible eardrum; the inner wall is mucous membrane-lined bone. Mucus is a slippery secretion that moistens and protects the underlying tissue. It acts like flypaper, trapping any germs that make it into the middle ear.
The middle ear contains three tiny bones, or ossicles, that form amoving bridge across the middle ear space. The outermost of these bones is the malleus (or hammer); it attaches to the tympanic membrane. The middle ossicle is the incus (or anvil); its body has two legs, which project at right angles to one another. One of these legs is connected to the malleus, and the other is connected to the third ossicle, called the stapes (or stirrup). The stapes rests on the oval window, a thin membrane on the inner wall. This chain of three bones receives sound waves from the eardrum and transmits and amplifies them through the oval window to the cochlea, the miraculous structure that transforms sound waves into nerve signals. From the cochlea, these signals travel on to the brain. A membrane covers all of the structures of the middle ear, including the ossicles. This glistening membrane constantly produces mucus.
Where does all of this mucus go?
There is only one way out of the bony-walled middle ear cave. Behind the ear is a bone called the mastoid process. As children grow, a labyrinth of tiny, connecting caverns forms in this bone and connects to the middle ear. Mucus can enter this labyrinth. Sometimes children get mastoiditis, an important complication of ear infections that we'll discuss in chapter 10. The mastoid air cells, though, are a dead end. The only way for mucus to exit the middle ear system is through the eustachian tube.
What is the eustachian tube?
As you will see throughout this book, the function (or dysfunction) of the eustachian tube is a critical element in ear infections. The eustachian tube is a drainage tube that goes from the middle ear to the back of the throat. The section closest to the ear is rigid; the section closest to the back of the throat is floppy.
What does the eustachian tube do?
This tube has three important functions. For precise hearing, the most important function is ventilation or pressure equalization. In order for the eardrum to vibrate optimally in, response to sound waves, the air pressure inside the middle ear space must be approximately equal to the air pressure in the external canal. The eustachian tube functions as a floppy valve that allows extra air to enter or to leave relative to pressure changes in the outside air. If the middle ear were a completely seated cavity, then rapid changes in middle ear pressure would cause the ear to burst like a balloon.
The second function of the eustachian tube is drainage of the secretions normally produced by the lining of the middle ear. The middle ear sports inucusproducing skin that is a continuation of the mucus lining of the eustachian tube, nose, and throat. Tiny hairs, or cilia, dot the surface of the eustachian tube. These energetic hairs move the secretions toward the back of the throat, where they are swallowed.
The third function of the eustachian tube is to protect the middle ear from bacteria, secretions, and pressure (while sneezing, for instance) from the nose and mouth. If the tube were wide open all the time, the tube would still be an effective drain, but it would not be an effective line of defense. The tube, then, typically remains closed at rest and opens only during swallowing or yawning. At these moments, the secretions drain, and the air pressure equalizes.
The proper functioning of the air-filled middle ear space allows us to hear clearly. Hearing is one of our most powerful ways of communicating with the world around us. When something occurs outside our direct line of vision, we may not be able to see the movements, but our hearing alerts us to the sounds. We become aware of events that occur at great distances and can locate sounds in space by subtle differences in the sound waves hitting our two ears. Perhaps most important, hearing opens a door to rich and varied speech, language, and music. It enables us to communicate with the people around us. Loss of hearing cuts us off from many fife experiences.
Such is the importance of the tiny anatomical structures we have discussed. The smallest bones in our bodies have a big effect on our lives. And it is the unseen, unglamorous eustachian tube that is the focus Of the struggle with ear infections.The Parent's Complete Guide To Ear Infections. Copyright © by Alan Greene. Reprinted by permission of HarperCollins Publishers, Inc. All rights reserved. Available now wherever books are sold.