Ear and hearing -the wonderful world of sound

Did you listen to your favorite music today? Have you ever wondered how we actually perceive sounds?

Without this nature given system what is the use of the most sophisticated man made stereo system.

The structure of ear is quite delicate and complicated. It is divided into 3 parts for the purpose of description.

The external ear consists of the pinna-the projecting part we see and the external auditory canal-a small passage leading from outside to the ear drum.

The shape of the ear helps in directing sound waves towards the ear canal. The pinna serves cosmetic purpose also. Imagine a face without ears. It also helps us to wear spectacles, and ear rings.

Structure of ear (please click on the graphic to see enlarged view)

At the inner end of the external auditory canal is situated the tympanic membrane (ear drum). The ear drum is oval in shape and separates the external ear from the middle ear. The middle ear is located in the temporal bone.

The middle ear contains three small bones called ossicles. From outside inwards they are malleus, incus and stapes.

The malleus is shaped somewhat like a hammer. It has a head, neck, handle (manubrium), anterior and lateral processes. The handle of malleus is attached to the tympanic membrane. The head of malleus articulates with a small concave area on the next bone in the chain called incus.

Left malleus. A. From behind. B. From within.

The incus consists of a body and two crura (bony projections) one short and the other long. The body forms a joint with the head of malleus. The long crus is connected to the head of stapes the last bone in the chain.

Left incus. A. From within. B. From the front.

The stapes is the smallest bone in the body. It is only 0.25 to 0.33 cm long (0.10 to 0.13 inches) and weighs only 1.9 to 4.3 milligrams.

A. Left stapes. B. Base of stapes, medial surface.

Apart from the three ossicles the middle ear also contains 2 muscles-tensor tympani and stapedius. Tensor tympani muscle arises from an area near Eustachian tube and is attached to the handle of malleus. When it contracts it pulls the ear drum inwards and keeps it taut. A loose ear drum will not be able to effectively transmit vibrations to the ossicles.

The stapedius is the smallest muscle in the body and one end in attached to the foot of stapes. It prevents excessive vibration of ossicles when exposed to very loud sounds.

The 3 ossicles and 2 muscles are packed in an area which is just 15 millimeters vertically and antero-posteriorly.

A tube called Eustachian tube connects the middle ear to the throat. It helps in equalizing pressure on both sides of the tympanic membrane by connecting the middle ear cavity with the atmosphere through the throat.

Normal hearing requires that the pressure on both sides of the tympanic drum is the same. If the pressure on one side is greater the ear drum will be pushed to the other side and will not be able to vibrate properly.

The inner ear is also called labyrinth because of its complicated structure (labyrinth means tangle). It is securely contained in the hardest bone in the body the petrous part of the temporal bone. It is made up of bony part and membranous part. The membranous part is situated inside the bony part.

The bony labyrinth consists of 3 parts-cochlea, vestibule and the semicircular canals which are 3 in number. It contains a clear fluid called perilymph.

Bony labyrinth

Interior of Bony labyrinth

The membranous labyrinth is inside the bony labyrinth and has the same form except in the vestibule. It consists of sac like structures and tubes. It contains a clear fluid called endolymph. It is separated from the bony wall by the perilymph.

Membranous labyrinth

The cochlea in so called because it is a coiled structure. It measures about 5 mm. from base to apex, and its breadth across the base is about 9 mm. It contains the nerve endings of the cochlear division of the eighth cranial nerve-the vestibule-cochlear.

The vestibule measures about 5 mm. from before backward, the same from above downward, and about 3 mm. across.

There are three semicircular canals called superior, posterior and lateral. They are concerned with the sense of balance. They open into the vestibule by five orifices one of the apertures being common to two canals.

The foot of stapes sits on an opening in the bony labyrinth called the oval window. The vibrations of the stapes cause movements in the fluid inside the cochlea. There are specialized cells in the cochlea which have fine hair like projections. These projections extend into the fluid in the cochlea.

The movements of the fluid cause the hair like projections to move back and forth. These movements result in generation of electrical impulses which are carried by the auditory nerve to the part of the brain responsible for interpreting these electrical impulses as sounds.

Common ear problems

• Ear infections in children-otitis media

Back to human body from ear and hearing