Michelle Longpree, age 12, of St. Augustine, Fla., for her question:
WHAT IS THE INNER EAR?
The part of the body we call the “ear” is only a portion of this vital organ. The ear extends deep into the skull. It has three main parts: the outer ear, the middle ear and the inner ear. Each plays an important part in hearing.
Let’s look at the inner ear after we examine the outer ear and the middle ear.
The outer ear is also called the pinna. It has two important parts: the auricle, which is the fleshy covered part attached to the side of the head, and the opening which is called the external auditory canal.
The auricle has a cuplike shape that can collect sound waves and direct them into the external auditory canal. The outer third of the canal is lined with fine hairs and with tiny glands that produce wax. The hairs and wax trap dust, insects and other particles.
The middle ear, or tympanic cavity is separated from the external auditory canal by the eardrum or tympanic membrane. At the other end, a thin wall of bone separates the middle ear from the inner ear. Three small, movable bones are linked to each other inside the tympanic cavity.
The malleus or hammer is attached to the eardrum. The middle bone, called the incus or anvil, connects the malleus and the innermost bone, the stapes or stirrup. The stapes attaches to the inner ear. Sound waves pass from the external auditory canal through the eardrum, across the three bones (malleus, incus and stapes), then to the inner ear.
The inner ear has so many intricate chambers and passageways that it is called a labyrinth. The inner ear is formed partly within the temporal bone of the skull. A fluid called perilymph separates a bony labyrinth from a membranous labyrinth. The membranous labyrinth itself is filled with a fluid called endolymph.
The inner ear has three connected parts: the vestibule, the semicircular canals and the cochlea.
The vestibule is the central part of the inner ear. One side of it connects with the middle ear. Above and behind the vestibule are the three semicircular canals, which give us our sense of balance. In front of the vestibule is the cochlea, which looks like a snail shell. It contains the sense organ for hearing, the organ of Corti.
A sound wave is a vibration in the air. The structure of the ear enables sound vibrations to pass from the outside of the head to the inner ear, and then to the part of the brain that controls hearing.
For a person to hear, the vibrations must reach the organ of Corti in the inner ear.
Sound waves are turned into nerve impulses inside the cochlea. This tube is about an inch and a quarter long. A membrane extends almost to the tip of the tube.
The cochlea’s membrane has the organ of hearing inside it. Sound waves cause the stapes to move in and out of the oval window, producing vibrations in the perilymph. These vibrations stimulate nerve endings of the organ of hearing. The nerves send messages to the brain.