Contents and Functions of Middle Ear
Dr.A.P.Ganesan
Komarapalayam
What are the contents and functions of middle ear
Structure of middle ear
The Tympanic Cavity
The middle ear or tympanic cavity is situated within the petrous part of temporal bone between the external and internal ear.
Lined by the mucous membrane
Filled with air
Communicates with nasopharynx through Eustachian tube
Communicates with mastoid air sinuses.
Epitympanic recess is situated superiorly.
Measurement
Vertically 15mm, Anteroposteriorly 15mm
Side to side -6mm in upper part,2mm in middle part ,4 mm in lower part
Boundaries
The middle ear has a roof, floor and four walls -anterior,posterior , medial and lateral walls
The roof is formed by thin plate of bone called tegmentympani of petrous part of temporal bone.
The floor is formed by a convex plate of bone of temporal bone .
Anterior wall consists of
1.An opening of canal of tensor tympani.
2.An opening of Eustachian tube .
3.A plate of bone called process cochleariformis.
4.Carotid canal.
Posterior wall
Formed by temporal bone with opening leading to mastoid antrum . Through which air passes to mastoid air cells
Medial wall
Made of thin layer of temporal bone in which there are two openings :-
1.Oval window (fenestra vestibuli ) 2. Round window (fenestra cochlea )
The oval window is occluded by ossicles (stapes) and round window is occluded by a fine sheet of membrane.
The lateral wall is made of tympanic membrane.
Auditory Ossicles
There are 3 small bones situated in the middle ear namely 1. Malleus 2. Incus 3.Stapes
Malleus
It resembles a hammer and has a head ,neck and 3 processes - 1.handle 3.anterior process 3. Posterior process . The lateral border of the head is firmly attached to tympanic membrane .The tendon of tensor tympanic is attached to the handle. The head articulates with body of incus.
Incus
It resembles premolar tooth in shape . It has a cubical body and two processes namely short and long process and lentiform nodule. The long process descends vertically parallel to the handle of malleus. Its lower end turns medially to end in lentiform nodule.
Stapes
It resembles stirrup and presents a head, neck and two limbs-anterior and posterior and base .
The head is directed laterally and there is a depression in it to articulate with lentiform handle of incus.The base is directed medially and closes the fenestra vestibuli to which it is fixed .
This 3 osssicles held in positon by ligaments
Tympanic Membrane (Ear drum or Tympanum)
It is oval in outline and is made of 3 layers - 1. Outer skin of external auditory meatus , 2. Middle fiberous layer and 3 inner mucous membrane of the middle ear .
It is vertically 10mm in length and horizontally 9mm in length. It is set obliquely making angle of 55 degree .It is curved with convexity towards the middle ear . the most convex part is called umbo.It presents a notch above. From the margin of the notch two mucous membrane fold arise and attach with the lateral process of malleus. The handle of the malleus is firmly fixed with inner surface of the tympanic membrane.
Functions of middle ear
1. Transmits the vibrations of the tympanum caused by sound waves to the inner ear
2. Keeps the pressure inside the middle ear in equilibrium with the atmosphericc air through eustachian tube so that the tympanum functions well
A loud sound causes reflex contraction of both the muscles in middle ear,tensor tympani and stapedius after a latent period of 40 t0 80 milli seconds .This is called tympanic reflex .
The tympanic reflex protects the tempanic membarane from being ruptured by loud sound .
It helps to protect the cochlea from damage by the loud sound
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