The ear is the sense organ that in vertebrates detects sound and helps maintain
balance. The visible portion of the ear, called the auricle,
pinna, or earflap, is part of the outer ear.
Together, the outer ear and the middle ear
collect sound and transmit it to the inner
ear which contains the organs of (except in fish) hearing
Main components of the human ear
- The pinna is formed of cartilage
covered with a layer of thin and flexible skin. In the human ear
it is not particularly important and makes only a small contribution
to the efficiency of hearing.
- The lobe of the ear.
- The external auditory meatus, or ear
canal is about 2.5 cm (1 inch) long. It is frequently contains
a number of coarse hairs, and the glands in its walls secrete
a waxy substance which dries and is sometimes shaken loose by
the head's movements.
- The tympanic membrane, or eardrum,
stretches across the inner end of the external auditory meatus.
In a healthy ear it is so thin that it is almost transparent.
- The auditory ossicles are
called the hammer, the anvil, and the stirrup, because they look
rather like these objects. You can see from the illustration above
how the handle of the hammer is attached to the tympanic membrane,
and how the base of the stirrup is attached to the opening called
the fenetra ovale, or oval window. The anvil joins the top parts
of these two bones together. When a sound reaches the ear, the
ear drum vibrates and this movement is transmitted through the
bones to the oval window. An interesting point is that the bones
are arranged so that quite small movements of the eardume cause
much larger movements of the oval window.
- The Eustachian tube
connects the cavity of the middle ear with that of the pharynx
(the cavity behind the nose, mouth, and the larynx). Its function
is to allow air to pass in and out of the middle ear and so to
keep it at the same pressure as the atmosphere outside. Unless
the pressures on both sides of the membrane are the same it cannot
- The fenestra ovule, or oval
window, occupies the little gap in the bone between the middle
and inner ears. It is filled completely by the base of the stirrup
and the small ligament surrounding it.
- The utricle and sacculus
are the middle parts of the membranous sac of the middle ear.
They lie in a part of the bony
labyrinth called the vestibule. Inside these two organs are
little ridges of tissue which change positions as the head is
tilted. Nerve impulses keep
the brain informed about the position of the head.
- The semicircular canals
lie at the back of the bony labyrinth. In order to show them clearly
in the illustration some of the bone of the labyrinth has been
left out. The semicircular canals are filled with a fluid called
endolymph. When the head moves, the fluid in at least one of the
canals lags behind a little and the pressure brought about by
this lag causes nerve messages to be sent to the brain. From these
messages the brain can tell which way and how fast the head is
- The cochlea is so named because
of its likeness to a snail (the Latin cochlea means "snail").
Its wider part is in contact with the oval window, and when the
base of the stirrup moves in and out the endolymph in the cochlea
moves too. As a result some of the little hairs within the cochlea
vibrate. From the bases of these hairs nerve impulses go along
the auditory nerve (11) to the brain, which integrates the intensity
and pitch of the sound that
is being received.
The only parts of the human ear one usually sees are the pinna and the outer
end of the external auditory meatus, or ear
canal. All the other parts of the ear lie hidden in a series of little
chambers hollowed out of the temporal
bone at the inner end of the meatus. In this way the delicate structures
are protected from injury, while at the same time being able to receive
sound waves from the outside.
Anatomists usually think of the ear as bring formed of three parts, the
outer, middle, and inner ears. The outer ear
consists of the pinna and the external auditory meatus. The function of
the meatus is to collect sound waves and channel them to the tympanic membrane,
or eardrum, at its inner end.
The middle ear is a tiny chamber which
contains the three little bones of the ear, the auditory ossicles. These
bones are so arranged that they transmit the vibrations of the eardrum to
the actual organ of hearing, the cochlea,
in the inner ear.
The inner ear consists of several intercommunicating
membranous sacs. These fit snugly into cavities in the temporal bone, which
are so complex in form that they are called the bony labyrinth. The sacs
are delicate sensory organs which not only enable us to hear but also give
information about the position and movements of the head.