It helps in analysing the pitch, quality, timbre, intensity, and direction of the sound waves. Moreover, the ears are related with totally unrelated function of maintaining balance and body equilibrium. Parts of ear : Human ear is divided into three parts:
- External ear,
- Middle ear, and
- Internal ear.
It is the outer part of the ear which is further differentiated into three parts :
(a) External pinna,
(b) External auditory canal and
(c) Tympanum or eardrum.
(a) External pinna
It is an oval, skin- covered flap that projects from the side of the head. It collects sound waves and directs these waves into the external auditory canal.
(b) External auditory canal
It is an oblique, S-shaped, 2.5 cm long tube. It extends from pinna upto the tympanic membrane. The oblique passage prevents the hard objects hitting the tympanum directly. In the beginning of this canal are present hair that act as strainer.
They do not allow the dust and other foreign particles to enter into the ear. In the internal lining are also present a large number of ceruminous or wax glands that secrete earwax.
Earwax lubricates the tympanum for proper functioning. In external auditory canal the combination of hair and wax prevents entry of dust particles into the ear.
(c) The tympanum or eardrum
It is a rounded flap of the diameter of a pencil. It is stretched tightly like a drumhead between the external and middle ear.
The function of external ear is to collect sound waves and convey it to the middle ear.
It is an air-filled, irregular space called tympanic cavity. It is an almost airtight cavity except that it opens into the pharynx through eustachian tube.
It acts as a ventilator to equalize pressure of air on both the sides of tympanic membrane. The inner boundary of middle ear is formed by a thin bone that bears two apertures covered by membranes.
The upper aperture is called oval window or fenestra ovalis and the lower aperture is called round window or fenestra rotunda.
Running across the middle ear from tympanum to oval window is present a more or less mobile chain of three small bones called ear ossicles.
These ear ossicles are outer malleus, middle incus and inner stapes. The malleus is hammer-shaped and Its handle is attached with the tympanum.
The incus is anvil-shaped. Whereas the stapes which is the smallest bone in human hody is stirrup-shaped.
It is attached with the membrane over oval window. The three ear ossicles form a leverage system. They transmit and amplify the sound waves from external to the internal ear.
It is represented by an irregular, very structure called membranous delicate labyrinth. It is fitted into a similarly shaped space called bony labyrinth in the temporal bone.
In-between the two, there is present a narrow space filled with a dense fluid called perilymph. The membranous labyrinth is also filled with a similar fluid known as endolymph. Membranous labyrinth consists of 3 parts :
(a) Semicircular canals
(a) Semicircular canal
These are three arched, semicircular canal present in 3 mutually perpendicular planes. Two of the three canals are vertical called anterior and posterior vertical semicircular canals and one is horizontal semicircular canal. All the three semicircular canals open into the utriculus (a part of vestibule).
One end of each semicircular canal is swollen forming an ampulla. In each ampulla there is present a sensory spot or sensory cells known as crista. The semicircular canals (cristae) help in dynamic equilibrium or dynamic balance while body is in motion.
It is the middle bag-like or sac-like structure. It is further differentiated into the following two parts : (i) Utriculus and (ii) Sacculus. Each part contains one sensory spot or sensory cells called macula which helps in static equilibrium.
It is spirally coiled like a snail’s shell having 22 coils. The cochlea is divided into three horizontal chambers called scalae upper (scala vestibuli), middle (scala media), and lower (scala tympani).
The three scalae are separated from each other by two membranes : upper Reissner’s membrane and lower Basilar membrane.
The upper and lower scalae are full of perilymph and the middle scala is full of endolymph. The basilar membrane has got sense organs of hearing known as organs of corti or spiral organs.
It consists of five longitudinal rows of sensory hair cells which serve as auditory receptors. The sensory cells transform the sound waves into nerve impulses. These are carried to the brain through auditory nerve.
Functions of ear
The ear performs two unrelated functions: hearing and equilibrium.
Sound produced causes vibrations in the air which travel as waves known as sound waves.
- The external pinna collects the sound waves. It concentrates these waves and passes on to tympanic membrane through auditory canal. The tympanic membrane starts vibrating in unison.
- These vibrations are conveyed to the membrane over fenestra ovalis through the ear ossicles.
- Vibrations in the membrane over oval window are passed on to the perilymph in the upper scala of cochlea.
- These vibrations are transmitted to the endolymph of middle scala and the basilar membrane starts vibrating.
- The vibrating basilar membrane stimu- lates the sensory (hair) cells of organs of Corti.
- The sensory (hair) cells convert the sound waves into nerve impulses. These are carried to the brain through auditory nerve.
- It is the brain which analyses the pitch, quality, timbre, intensity and direction of the sound waves.
(a) Dynamic equilibrium : Semicircular canals help in dynamic equilibrium. When the head turns in different directions the fluid inside the semicircular canal called endolymph is also shaken.
This moving fluid pushes against sensory cells (cristae) sending nerve impulse to the brain through nerve fibres attached to them via auditory nerves.
(b) Static equilibrium : Utriculus and sacculus (Vestibule) helps in static equilibrium. The sensory cells (maculae) helps in to register static (positional) balance with respect to the gravity.
CAUSES OF DEAFNESS OR HEARING IMPAIRMENT
Hearing impairment or deafness may be due to :
- The rupturing of tympanum or eardrum.
- The blockade of external auditory canal.
- The damage of auditory nerve or cochlea.
- Excessive growth of ear ossicles.
- Blockade of sound waves in the external or middle ear due to any reason.