The ear can be divided into three basic areas (Figure 1). The outer ear refers to the ear canal that is approximately one inch in length and ends at the level of the eardrum (known as the tympanic membrane). The eardrum identifies the beginning of the middle ear, which is a cavity in the bone of the skull that houses the three bones of hearing (known as ossicles). The bone completely surrounding the ear structures that forms a large part of the skull base is known as the temporal bone. The first bone of hearing is attached to the underneath side of the eardrum and is shaped somewhat like a hammer. This bone is called the malleus. The second bone is in the shape of an anvil, and is the incus. The third bone of hearing, known as the stapes, is shaped like a stirrup. These three bones are connected to one another and have the ability to vibrate, or move back and forth. The base of the stapes bone (known as the footplate) is situated at the junction of the middle and inner ear. The inner ear is a separate cavity or space deeper in the temporal bone but, unlike the middle ear, is fluid-filled. This structure consists of a series of curved channels or chambers that can, in turn, be divided into two basic components. The cochlea is shaped like a snail shell and is the part of the inner ear concerned with hearing. The semicircular canals (of which there are three) are shaped like arcs or loops and are responsible for the balance function of the inner ear. On the deep aspect of the inner ear the nerves of hearing and balance exit and travel to the brain.

    When sound waves enter the ear canal, they are directed down to the eardrum. These pressure waves then hit the eardrum, causing it to vibrate. This, in turn, leads to vibration of the ossicles. The footplate of the stapes bone is located over an opening into the inner ear and forms a seal that allows for some movement. When the footplate vibrates, it causes movement of the fluid within the cochlea of the inner ear. This fluid movement is detected by very delicate hair cells and nerve endings within the inner ear. From there, the information travels as electrical impulses along the nerve of hearing to the brain.

    There are two types of hearing loss - conductive and sensorineural. A conductive hearing loss is due to a problem in the outer or middle ear. This may be due to obstruction of the ear canal, or a problem with the mobility of the eardrum or the ossicles. A sensorineural hearing loss is due to a problem in the inner ear or the nerve of hearing. An audiogram (hearing test) can readily differentiate between the two types of hearing loss, and determine the degree of the loss.
   
    The balance mechanism of the inner ear responds to head position. With any type of movement, the fluid in the semicircular canals is set into motion. This is detected by the hair cells and nerve endings of the balance nerve, leading to electrical impulses within the nerve that are transmitted to the brain.