The Bones of the Ear

The outer ear is the part most people are most familiar with and which collects sound. Sound waves are funneled by the auricle or pinna, made up of flexible cartilage. The waves the travel down the auditory canal and on to the eardrum. This zone of the ear contains no bones but is a vital part of the complicated process of sound collection. Damage or loss of any part of the outer ear can adversely affect the quality of the sound waves passed on to the inner zone, likely lead to hearing difficulties or even hearing loss.

The middle ear contains three tiny, connected bones called ossicles, as well as the eardrum which is not a bone. In order, these small bones are the

• malleus,
• incus, and
• stapes.

The ossicles are the smallest bones in the human body, and they serve to amplify collected sound and concentrate it in a small area. This makes the transmission of sound onto the inner zone of the ear more accurate.

The malleus or hammer has a long handle that attaches to the eardrum at one end. Despite its name, the shape of the malleus more resembles a club. Its purpose is to receive the vibrations from the eardrum and transmit them onward to the incus.

In this picture, the malleus is brown although, in reality, it is a creamy white color like all living bone. Inside the adult human ear, the malleus measures about 8 millimeters long and 3 millimeters wide, but it is not the smallest bone.

Often called the anvil, the incus is a bone between the malleus and the stapes. It acts as a bridge, transmitting vibrations further into the ear. While it attaches to the stapes by connective tissue, the top of the malleus and the body of the incus are held together by a tightly fitting joint. The incus is slightly larger than the malleus -- 9 millimeters long and 5 millimeters across in most adults.

Sometimes called the stirrup due to its similarity to a riding stirrup, the stapes is the smallest bone in the human body, usually measuring less than 3 millimeters across. Acting as the footplate, some people compare the stapes to a tuning fork, as both absorb and transmit vibrations into sound.

The two branches of the stapes transmit sound vibrations to the small, flat base of the bone. All the ossicle bones of the middle ear are connected, but the stapes is not in direct contact with the inner ear. Instead, the movement of the vibrations from the stapes enter the inner ear, where they are processed into information the brain can understand.

This final section of the ear includes several soft tissues. One is the oval window that connects the middle ear to the inner ear. Also present are the semi-circular ducts filled with fluid that send information on head position to the brain. Next is the auditory tube, which drains fluid from the middle ear into the throat, behind the nose. In this region, only the cochlea is made up of bone cells.

The cochlea is a snail-shaped organ that transforms sound into signals sent on to the brain. In the picture it is blue, but it is really pale gray and brown. Although it is not a bone, bone cells form part of the cochlea, and some medical textbooks refer to the cochlea as a bone. This organ is the final part of the ear and connects to the brain via the auditory nerve.

This tiny organ, measuring about 5 millimeters across, contains thousands of sensory cells or hair cells. Cochlear fluid vital to the process of hearing and maintaining our sense of balance surrounds these cells. If you spin around quickly, it is the swirling motion of the fluid inside your cochlea that makes you feel dizzy.