ii. The Middle Ear:
The middle ear is a small air-filled cavity. It is separated from the external ear by the ear-drum (tympanic membrane) and from the internal ear by a thin bony partition which contains two small openings namely, oval window and round window. The middle ear contains three tiny bones called ear ossicles. These bones are named as malleus (or hammer), incus (or anvil) and stapes (or stirrup).
The handle of malleus is attached to the internal surface of the eardrum (tympanum). Its opposite end is connected with the incus. The incus is the intermediate bone and it articulates with the head of the stapes.
The flat part of stapes fits into the oval window. Directly below the oval window is another opening called round window. The round window is covered by a thin membrane.
The anterior wall of the middle ear contains an opening that leads directly into the auditory tube (also known as Eustachian tube). This connects middle ear with the throat thus any throat infection may lead to the ear through the Eustachian tube. The Eustachian tube also helps in equalizing air pressure on both sides of the eardrum.
iii. The Internal Ear:
The internal or inner ear is also called membranous labyrinth.
It has two main parts—cochlea and semicircular canals. Cochlea is a hollow, spiral-shaped (coiled), chamber. It resembles a snail’s shell. It consists of a bony spiral canal that makes about 23/4turns around a central bony core. Its inner spiral cavity contains three separate channels or canals that run parallel.
These canals are separated by membranes. The median (cochlear) canal is filled with endolymph and the outer two canals are filled with a fluid called perilymph. The middle canal contains a spiral organ called the organ of Corti. The organ of Corti is the organ of hearing. It contains a series of nerve cells and hair cells which join the auditory nerve and help in hearing.
The hair cells of the spiral organ are very sensitive to sound. They can be damaged by exposure to high-intensity noises such as noise produced by engines of jet planes, loud music, etc. The inner ear also contains three semicircular canals. These canals are arranged at right angles to each other in three different planes (one horizontal, two vertical) and is filled with endolymph. One end of each canal is swollen to form an ampulla. The ampulla contains sensory cells which help in balance of the body while moving. The nerve fibres arise from these cells and join the auditory nerve.
There is a short stem (vestibule) which joins the semicircular canals and cochlea. It contains two small sacs. The utriculus and sacculus. These also contain tiny hair like sensory cells which help in static balance of the body while at rest (i.e. position of head when not in motion like standing). The external ear, middle ear and cochlea help in hearing. The sacculus, utriculus and the semicircular canals help in the sense of balance.
Summary of various parts of ear and their functions.