The Ear: The Marvelous Organ of Hearing
The ear is divided into three parts:
(1) The external ear, made up of the outer portion and passage-way which leads up to the drum.
(2) The middle ear or drum, the continuation of the ear passage internal to the drum membrane, and
(3) The internal ear containing the labyrinth and the nerve of hearing.
Description of the External Ear
The outermost part, the skin-covered auricle, contains no bone, being simply a mass of cartilage covered by skin. It acts as a sound catcher and improves the hearing by directing sound-waves into the opening or external meatus. This meatus or passage-way runs directly inward for an inch and a half. The inner half of the passage-way runs through solid bone, ending abruptly at the membrane or sounding-board of the ear.
THE JOURNEY OF SOUND WAVES TO THE BRAIN
This diagram shows the marvelous structure of the ear, and how sound reaches the brain. There is marked similarity between the ear and a telephone receiver by which we are able to receive messages from the outside world. Hearing is simply the result of sound-waves striking the drum of the ear which set in vibration the bones of the middle ear, and they in turn vibrate the drum of the inner ear. This sets in motion a fluid, and the wave motions are conveyed along the spiral staircase to the wires, or nerves of hearing, and from there to the telephone exchange, or brain.
Large illustration (398 kB)
Description of the Middle Ear
This part begins at the inner surface of the membrane, and extends inward for about a quarter of an inch. The outer surface of the membrane can be seen by the observer on pulling the top of the auricle or fleshy part of the ear a little upward, so as to straighten out the somewhat curved passageway or meatus. The membrane which is placed transversely across the meatus is whitish-pink or yellowish color.
What the Middle Ear Contains
The chief contents of the cavity of the middle ear are three tiny bones called the malleus or hammer bone, the incus or anvil bone, and the stapes or stirrup bone. In addition, an important nerve called the chorda tympani passes across the middle ear chamber. The three little bones contained in the middle ear may be looked upon as the connecting link between the outer ear, which gathers the sounds, and the internal ear, which transmits the effect of the sound waves to the brain, where they are translated into what we call hearing.
From without inward the three little bones lie touching each other, end to end, the outer end of the first bone being implanted between the layers of the drum membrane and the inner end of the innermost bone, fitting into a tiny opening which connects the middle ear with the internal ear. As the result of their lying touching each other, any movement of the ear drum caused by a sound wave striking against its outer aspect, moves the malleus bone; this, in turn, moves the middle incus, and this passes the movement on to the innermost part of the stirrup. This, in turn, passes the movement onward to the fluid or perilymph in the outermost part of the internal ear, and here the endings of the nerve of hearing receive the stimuli which we recognize as “sounds.” (See Plates.)
The Two Important Tubes of the Middle Ear
In addition to these contents of the middle ear there are also two tiny openings which, very necessary for health, are nevertheless sometimes a pathway by which serious disease may attack the ear and destroy the hearing. The first is a small passage-way leading from the upper part of the middle ear cavity through the bone to the mastoid antrum, a hollow space in the prominent mass of bone to be felt immediately behind the ear projecting outward and downward from the skull.
The second passage-way opening into the middle ear cavity is that of the Eustachian tube which leads directly to the back of the throat. The importance of this tube is that through it air can find its way directly into the middle ear, so that the air pressure on the two sides of the drum is always kept the same. If it were not for some such arrangement the pressure on the outer side of the drum would become greater than that on its inner surface. This would, of course, push the drum inward, and greatly reduce its mobility.
Explanation of the Internal Ear
This is a complicated structure of bony passages curled on themselves, roughly as in a snail shell, and lined with a delicate membrane. This membrane is, so to speak, floating in fluid. The layer of fluid between it and the bone is called the perilymph, while the two layers of the membrane enclose a similar fluid termed the endolymph. The internal ear or membranous labyrinth may be divided roughly into three chief parts: (1) the cochlea, the true organ of hearing; (2) the semi-circular canals, which control the act of balancing; and (3) the vestibule, or introductory chamber to the semi-circular canals.
The cochlea is a collection of three tubes curled up on themselves in snail-shell fashion.
The central canal of these three is the connecting link by which the sound waves, passed along over the three tiny bones—the malleus, incus, and stapes—finally reach the endings of the main nerve of hearing, the auditory nerve. (See Plate.)