The Hearing System
The outer ear: That part of the ear that we can see. It is made up of two parts - the ear flap (pinna) and the ear canal. This is the part of the system that directs sound into the middle and inner ear. It also provides protection for the more delicate parts of the hearing system.
The middle ear: It starts with the ear drum (typmpanic membrane), which is located at the end of the ear canal. Sound waves, coming through the air, enter and travel through the ear canal to strike the ear drum and cause it to vibrate. These vibrations are picked up and continued by a small chain of bones (ossicles or the ossicular chain). The first bone in this chain is the hammer (malleus), and it is attached to the ear drum. The second bone is the anvil (incus). The third bone is called the stirrup (stapes) which is attached to the inner ear at an oval window in the bony capsule of the inner ear. It is necessary for these bones to act as a series of levers in getting the vibrations to move the rather stiff fluid in the inner ear. The vibrations, started outside and in air, continue along this chain of bones and are then transmitted to the fluid of the inner ear. The eustachian tube is a part of the middle ear because it connects with the throat to allow for drainage and air exchange. This keeps the pressure of the middle ear equal to the pressure of the outside air.
The inner ear: It has two connected parts - three loops are called the semicircular canals and they assist us in keeping our balance (vestibular system); the snail shell portion is the cochlea, which is the sensory organ of the hearing system. It is the cochlea that contains the fluid which is moved by the vibration of the oval window. This movement activates the tiny hair cells within the cochlea to fire off or send electrical signals through the auditory nerve to the hearing centers of the brain.