The human ear can be divided into 3 parts – external, middle and internal – with each part playing an integral role in the sense of hearing, while the internal ear has an added function for equilibrium. The external (outer) and middle ear transmit sound waves to the internal (inner) ear. Here mechanical sound waves are converted into electrical impulses which are conveyed to the brain for processing. The vestibulocochlear organ within the internal ear is also responsible for equilibrium and maintains the sense of balance.
The external ear (outer) is made up of the auricle, ear canal and lateral surface of the tympanic membrane. Its function is to trap sound waves (auricle) and transmit it to the inner ear by passing down the canal and causing the eardrum to vibrate.
Picture of the Human Ear from Wikimedia Commons
The outer shell-shaped part of the external ear is known as the pinna or auricle. It traps sounds waves in the surroundings and directs it into the ear canal. The auricle is composed of elastic cartilage covered by a thin layer of skin. The lower part known as the lobule (common name ~ ear lobe) is made up of fibrous tissue, fat and blood vessels.
The outer margin of the ear is known as the helix and the inner elevated margin is the antihelix. The deepest depression which leads to the ear canal is known as the concha. The tragus is the small cartilaginous flap that can be pushed down to block the opening to the ear canal.
The ear canal runs from the concha to the ear drum (lateral side of the tympanic membrane) and is known as the external acoustic meatus. It runs inward through the temporal bone of the skull (tympanic part) and is about 2 to 3 centimeters long,
The ear canal is an S-shaped tunnel. The outer one-third is made up of cartilage lined with skin that is similar to the skin of the auricle. Sebaceous and ceruminous glands in this outer one-third produces earwax (cerumen). The inner two-thirds are bony and lined with a thinner skin.
The eardrum (tympanic membrane) divides the external ear from the middle ear. It is a thin membrane that is about 1 centimeter in diameter. The lateral part of the tympanic membrane which faces the ear canal is lined with thin skin that is continuous with the skin of the inner two-thirds of the ear canal.
The best description for the eardrum is that it looks like a satellite dish. It is concave so through the ear canal it looks like the back of a satellite dish, with a central depression known as the umbo.
The tympanic membrane moves inward and outward in response to vibration, similar to speaker. Due to the auditory ossicles, which are attached to the medial surface of the eardrum, the movement of membrane transmits force to the internal ear where it can be converted into electrical impulses and passed to the brain.
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.