EAR (Stato - Acoustic organ)


All the vertebrates have one pair of ears back to the eye 
 EAR = PHONORECEPTOR = STATOACOUSTIC ORGAN



Functions of ear

(1) To receive sound waves (hearing) 
(2) To maintain body balance.

The main function of the ear is to maintain the balance of the body.

Structurally, the ear may be divided into three parts :-  

  1.  External ear
  2.  Middle ear
  3.  Internal ear

Structure of Ear


EXTERNAL EAR 


It is the outer part of the ear. It is well developed in mammals only.
External ear is further divided into 2 parts:- 
1. Ear pinna 
2. Ear canal


Ear pinna 


These may be small or large, fan-like structures, an important feature of mammals. The skin of the ear pinna is hairy and has yellow elastic cartilage. Humans can not move their pinna. Muscles of man's ear pinna are vestigial. The pinna collects the vibrations in the air which produce sound.

EAR CANAL


The ear canal or external auditory meatus is a 24 mm long canal that is expanded from the base of the pinna to the inner side. Apart from mammals, birds and reptiles also have ill or less developed ear canals.

  • Ceruminous gland 
In the wall of the external auditory meatus or ear canal, there are found modified sweat glands (ceruminous glands) and sebaceous glands. These secrete cerumen or ear wax, which moistens the eardrum and protects it. 

MIDDLE EAR 

Tympanic membrane 

At the end of the ear canal a stretched, thin, obliquely placed membrane is present, it is called the eardrum or tympanic membrane. It separates the ear canal from the middle ear. It is the part of the middle ear.


Tympanic Bulla

The middle ear is also called the tympanic cavity. It is filled with air. This cavity is covered by a flask-like bone called a tympanic bulla. This bone is a part of the temporal bone of the skull 


Eustachian duct  

The middle ear cavity is connected by the pharyngeal cavity through a canal called the Eustachian tube. The Eustachian tube helps in equalizing the pressures on either side of the eardrum.


The tympanic cavity is connected by the internal ear cavity by two apertures:- 
(i) Oval aperture fenestra ovalis (oval window) 
(ii) Spherical aperture fenestra rotundus (round window). 
A thin and firm membrane covers each aperture. 

Ear ossicles  

Three ear ossicles (small bones) are present and arranged in a chain with movable joints connected together in the tympanic cavity. These ear ossicles are:-
1. Malleus 
2. Incus 
3. Stapes


Malleus

  • Hammer shaped
  • Modification of articular bone

Incus

  • Anvil shaped
  • Modification of Quadrate bone 

Stapes

  • Stirrup shaped
  • Modification of Hyomandibular bone
  • The smallest bone of the body 

Malleus and incus are Joint together by a synovial hinge joint. Incus is joined by stapes by ball and socket joint.

INTERNAL EAR 

The fluid-filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths. The bony labyrinth is a series of channels. Inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph. The membranous labyrinth is filled with a fluid called endolymph.





HEARING [Cochlear Apparatus] 

Cochlea

It is the sensory structure for hearing. It is a coiled (Length 2¾ coiling) structure present below the vestibular apparatus. The cochlea in cross-section show 3-canals:- 
1. Scala vestibuli - It is situated on the dorsal side and is filled with Perilymph. 
2. Scala media - It is situated between the scala vestibuli and scala tympani. It is filled with Endolymph. 
3. Scala tympani - It is situated at the ventral side below the scala media. It is filled with Perilymph.

The membrane of the roof of scala media which separates scala vestibuli from scala media is called Reissner's membrane. The membrane at the floor of scala media, which separates scala media from scala tympani is called Basilar membrane.  At the base of the cochlea, the scala vestibuli ends at the oval window, while the scala tympani terminates at the round window which opens to the middle ear.


License to NCERT

Path of Hearing 









THANK YOU FOR READING THIS