The middle ear is an air-filled cavity that lies between the outer ear and inner ear. It separated from the outer ear by the tympanic membrane (eardrum) and makes contact with the inner ear at the basal turn of the cochlea, round and oval windows.
Walls of the Middle Ear
The middle ear is somewhat box shaped and has six walls :
- Tegmental wall – roof (top)
- Jugular wall – floor (bottom)
- Membranous wall – lateral (outer)
- Labyrinthine wall – medial (inner)
- Mastoid wall – posterior (back)
- Carotid wall – anterior (front)
Three minute bones (auditory ossicles) within the middle ear – malleus, incus and stapes – transmit vibrations from the eardrum, caused by sound, to the inner ear.The differences in the size and orientation of the ossicles increases the force but decreases the amplitude of the vibrations.
The neck and handle of the malleus (hammer) connects to the tympanic membrane and the head of the malleus articulates with the incus.
The incus (anvil) articulates with the hammer on one end and the long limb of the incus articulates with the stapes at the other end.
The stapes (stirrup) is the smallest ossicle which articulates with the incus at one end and the base connects to the oval window.
The oval window transmits vibrations from the stapes to the cochlea of the inner ear. This allows the sound waves to be converted into electrical impulses which is the transmitted to the brain. The round window is separated from the middle ear by a membrane. It allows for the transmission of force within the cochlea by vibrating in response to mechanical waves (opposite phase to the oval window).
The eustachian tube, pharyngotympanic tube, connects the middle ear to the nasopharynx (the part of the the throat that communicates with the nasal cavity). This ensures that the air pressure of the environment which enters through the ear canal is equal to the air pressure within the middle ear.
If the pressure of the environment is higher than within the middle ear, the tympanic membrane will bulge inward. Conversely, if the pressure within the middle ear is higher than the environment, then the eardrum will bulge outwards. This can affect the transmission of sound, cause pain and may even lead to a rupture of the tympanic membrane. Fluid accumulation within the middle ear (effusion) can also push the tympanic membrane outwards.