Anatomy and physiology of Human Ear
It is an organ for hearing and maintenance of body Equilibrium. It is controlled by the brain through the eighth cranial nerve called vestibulocochlear nerve.
Size of Ear
These is no specific size number of ear it comes with different sizes and shapes. peoples from different Region have different size and shape.
According to journal Plastic and Reconstructive Surgery mens have slightly larger ear size than women’s. An average size of ear is 2.5 inches (6.3 centimeters) long, and the average ear lobe is 0.74 inches (1.88 cm) long and 0.77 inches (1.96 cm) wide.
Division of parts of Ears:
1. External Ear.
2. Middle Ear.
3. Internal Ear.
1. External Ear :
This is the outer most part of the ear, distinguished into pinna and auditory canal ( External acoustic meatus ).
a. Pinna (Auricle)
The Auricle is the expanded portion that project from the side of the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged ; the most prominent outer ridge is the helix. The lobule ( ear lobe ) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood. The pinna collects and directs sound waves into external auditory meatus..
b. External Auditory meatus ( Auditory canal )
Pinna opens into external auditory meatus ( canal ) supported by bone. This is a slightly ‘S’– shaped tube about 2.5 cm long extending from the Auricle to the tympanic membrane ( ear drum ). The meatus is lined with skin continuous with that of the auricle. Hair follicles and ceruminous gland (|wax gland ) are present inside the long canal. Ceruminous gland secrete a waxy substance the cerumen ( ear wax ). Ear wax prevents the foreign bodies ( insects, dust particles etc ) entering the ear..
2. Middle Ear :
External auditory canal is separated from middle ear ( tympanic cavity ) by a tympanic membrane or ear drum. It consists of ;
a. Tympanic cavity.
b. Auditory tube
c. Ear ossicles.
a. Tympanic cavity :
This is a cavity of middle ear filled with air. The tympanic cavity is connect ed with auditory capsule by two apertures:
1. Fenestra ovalis or Fenestra vestibuli or oval window or vestibular window.
2. Fenestra rotunda or Fenestra tympani or round window or cochlear window.
b. Eustachian Tube or Auditory Tube
It connects tympanic cavity with pharynx and serves to keep an qual air pressure on both sides of tympanic membrane, permitting the membrane ti vibrate freely with the sound waves.
The external pressure on tympanic membrane increases while diving and decreases on being lifted in air.
c. Ear Ossicles :
These are three very small bone that extend across the middle ear from the tympanic membrane to the oval window. They form a series of moveable joints with each other and with the medial wall of the cavity at the oval window. They are named according to their shapes..
i. The Malleus :
This is the lateral hammer – shaped bone. The handle in contact with the tympanic membrane and the head form a moveable joint with the incus..
ii. The Incus :
This is the middle anvil – shaped bone. Its body articulates with the malleus, the long process with the Stapes,and it is stabilized by the short process. Fixed by fibrous tissue to the posterior wall of the tympanic cavity..
iii. The Stapes :
Thisbis the medial stirrup shaped bone. Its head articulate with the incus and its footplate fits into the oval window. The three ossicles are held in position by fine ligaments.
3. Internal Ear :
This is situated within the petrous part of the temporal bone.
The internal ear contains :
a. Bony labyrinth ” Which consists of a series of channels ( present in the petrous portion of temporal bone lined with periosteum ) the bony labyrinth contains a fluid called ” perilymph” it is longer than membranous labyrinth, which fits into it, like a tube wuthin a tube.
b. Membranous labyrinth which lies within the bony labyrinth. The membranous labyrinth is filled with a fluid called “Endolymph”.
The bony labyrinth contains three structure :
a. Vestibule, which is situated posteriorly.
b. Cochlea (the oragan of hearing) which is situated anteriorly.
c. Semicircular canals (organ of equilibrium).
a) Vestibule :
It is the central part. It lies between cochlea in front and semicircular canals behind. It contains “utricle” and “saccule” which are the parts of the membrane labyrinth.
b. Cochlea :
It is a spiral canal which looks like the shell of a snail.
1. Modiolus is a central column of spongy bone around which the spiral canal twines.
2. Basilar membrane is a membrane septum which divides the cochlea into two part : the upper part and the lower part. The upper part is called “Scala vestibuli” and the lower part is called “Scala media“.
3. Organ of corti is the auditory receptor which rests on the basilar membrane. The organ of corti contains rows of elongated hair cell. The fibtes of cochlear nerve are in contact with these hair cells.
C. Semicircular canals :
Each ear has three semicircular canals which are placed at right angle to each other. They are posterior, superior and lateral semicircular canals. Each semicircular canal has an enlarged end called “Ampulla” the Ampulla has ending of vestibular nerves and also some hair like projections..
Functions of the ear :
1. Mechnism of hearing
The ear conduts sound waves from air to bone, then to fluid and finally transmits to the auditory centre of the brain.
2. Mechnism of equilibrium
Movement of head or alteration in its position produce movement of endolymph present in the semicircular canals. The movement of Endolymph stimulate the nerve endings in the ampullae. The impulses are carried to brain through the vestibular portion of the 8th cranial nerves. These impulses produces sensation which make us conscious about the position of the head. If the position of the head is disoriented, we can then adjust its so as to maintain balance and equilibrium.
Physiology Of Hearing :
1. Every Sound Produces sound waves or vibration in the air, travel at about 332 meters ( 1088 feet ) per second.
2. The Auricle, because of its shapes, concentrates the waves and directs them along the auditory meatus causing the tympanic membrane to vibrate.
3. Tympanic membrane vibration are transmitted and amplified through the middle ear by movement of the ossicles.
4. At their medial end the footplate of the Stapes rocks to and fro in the oval window, setting up fluid waves in the perilymph of the Scala vestibuli.
5. Some of the force of these waves is transmitted along the length of the Scala vestibuli and Scala tympani, but most of the pressure is transmitted into the cochlear duct.
6. This causes a corresponding wave motion in the endolymph, resulting in vibration of the basilar membrane and stimulate of the auditory receptor in the hair cells of the spiral organ.
7. The nerve impulses generated pass to the brain in the cochlear (auditory ) portion of the vestibulocochlear nerve ( 8th cranial nerve ).
8. The fluid waves is finally expanded into the meddle ear by vibration of the membrane of the round window.
9.The vestibulocochlear nerve transmits the impulses to the nearing area in the cerebrum where sound is preceived and to vsrious nuclei in the pons and the midbrain.
10. Sound waves have properties of pitch and volume or intensity pitch is determined by the frequency of the sound waves and is measured in hertz ( Hz ).
11. The volume depends on the amplitude of the sound waves and is measured in decibels ( dB). Very loud noise is damaging to the ear, particulatly when prolonged because it damages the sensitive hair cells of the spiral organ.
12. Because of The structure of the inner ear, sound of different frequency stimulate the basilar membrane at different places along its length allowing discrimination of pitch.
13. Additionally, the greater the amplitude of the wave created in the Endolymph, the greater the stimulation of the auditory receptor in the hair cell in the spiral organ, enabling perception of volume.