The middle ear is the air-filled central cavity of the ear. It is separated from the external ear by the tympanic membrane, or eardrum, and from the cochlea, or inner ear, by a membrane with an opening in the center called the oval window. It transmits external acoustic sound to the fluid in the cochlea, then makes adjustments so that the change in environment will not dilute the sound. It also helps to provide ventilation to the back of the throat and nose.
The tympanic membrane is adjacent to three small bones that together are called ossicles. The hammer-shaped malleus bone is connected to the anvil-shaped incus. The incus is connected to the stirrup-shaped stapes bone, which is the smallest bone in the human body. Together, the three bones are known as the ossicular chain.
When sound hits the tympanum, it starts a vibration that reverberates through the ossicular chain. The base of the stapes bone is connected to the oval window membrane. The sound transmitted by the ossicles vibrates through the oval window to the cochlea.
This sequence allows proper transference of sound pressure from the middle ear to the cochlea. It serves as a pressure amplifier for the sound waves. The process allows sound to enter into the fluid environment of the inner ear.
Another key part of middle ear function is the eustachian tube, a tube made of cartilage and membrane that goes from the middle of the ear cavity to the nasopharynx, or the nose and back of the throat. The eustachia tube also provides a place for air pressure to be equalized between the throat and the middle ear. A healthy eustachian tube is the pathway that provides protection and nourishment to all of the structures in the middle ear. The eustachian tube passes through the temporal bone of the skull and reaches to the mastoid lines, which serve as a site for muscle attachment.
Near the nasopharynx, the cells of the eustachian tube are lined with fine hair-like projections called cilia. As the membrane extends through the middle ear, the cells change to a tall columnar structure. After it reaches the mastoid, the membrane is composed of closely packed, cube-shaped cells.
Ventilation is another important function of the middle ear. The eustachian tube opens and closes as needed to allow air into the middle ear cavity. When the dilator tubae muscle is contracted, the tube opens. Improper ventilation can lead to a cleft palate, tumors, neuromuscular defects and craniofacial trauma.