Middle Ear

 Features

• The middle ear is also called the tympanic cavity, or tympanum.
• The middle ear is a narrow air filled space situated in the petrous part of the temporal bone between the external ear and the internal ear 

Shape and Size

• The middle ear is shaped like a cube. 
• Its lateral and medial walls are large, but the other walls are narrow, because the cube is compressed from side to side. 
• Its vertical and anteroposterior diameters are both about 15 mm. 
• When seen in coronal section the cavity of the middle ear is biconcave, as the medial and lateral walls are closest to each other in the centre. 
• The distances separating them are 6 mm near the roof, 2 mm in the centre, and 4 mm near the floor

Parts

• The cavity of the middle ear can be subdivided into the tympanic cavity proper which is opposite the tympanic membrane; and 
• the epitympanic recess which lies above the level of the tympanic membrane.

Communications

• The middle ear communicates anteriorly with the nasopharyrnx through the auditory tube, and 

• posteriorly with the mastoid antrum and mastoid air cells through the aditus to the mastoid antrum 

CONTENTS

The middle ear contains the following.

1. Three small bones or ossicles namely the malleus, the incus and the stapes. 

The upper half of the malleus, and the greater part of the incus lie in the epitympanic recess.

2. Ligaments of the ear ossicles.

3. Two muscles, the tensor tympani and the stapedius.

4. Vessels supplying and draining the middle ear.

5. Nerves: Chorda tympani and tympanic plexus.

6. Air.

• The mucous membrane lining the middle ear cavity invests all the contents and forms several vascular folds which project into the cavity. 
• This gives the cavity a honeycombed appearance.

BOUNDARIES

Roof or Tegmental wall

1. The roof separates the middle ear from the middle cranial fossa. 

• It is formed by a thin plate of bone called the tegmen tympani. 
• This plate is prolonged backwards as the roof of the canal for the tensor tympani.

2. In young children, the roof presents a gap at the unossified petrosquamous suture where the middle ear is in direct contact with the meninges. 

• In adults, the suture is ossified and transmits a vein from the middle ear to the superior petrosal sinus.

Floor or Jugular Wall

• The floor is formed by a thin plate of bone which separates the middle ear from the superior bulb of the internal jugular vein. 
• This plate is a part of the temporal bone.
• Near the medial wall, the floor presents the tympanic canaliculus which transmits the tympanic branch of the glossopharyngeal nerve to the medial wall of the middle ear.

Anterior or Carotid Wall

• The anterior wall is narrow due to the approximation of the medial and lateral walls, and because of descent of the roof.
• The uppermost part of the anterior wall bears the opening of the canal for the tensor tympani.
• The middle part has the opening of the auditory tube.
• The inferior part of the wall is formed by a thin plate of bone which forms the posterior wall of the carotid canal. 
• The plate separates the middle ear from the internal carotid artery. 
• This plate of bone is perforated by the superior and inferior sympathetic caroticotympanic nerves and the tympanic branch of the internal carotid artery 
• The bony septum between the canals for the tensor tympani and for the auditory tube is continued posteriorly on the medial wall as a curved lamina called the processes cochleariformis. 
• Its posterior end forms a pulley around which the tendon of the tensor tympani turns laterally to reach the upper part of the handle of the malleus.

Posterior or Mastoid walI

The posterior wall presents these features from above downwards.

1. Superiorly, there is an opening or aditus through which the epitympanic recess communicates with the mastoid or tympanic antrum.

2. The fossa incudis is a depression which lodges the short process of the incus.

3.  A conical projection, called the pyramid, lies near the junction of the posterior and medial walls. 

It has an opening at its apex for passage of the tendon of the stapedius muscle.

4. Lateral to pyramid and near the posterior edge of the tympanic membrane, is the posterior canaliculus for the chorda tympani through which the nerve enters the middle ear cavity

Lateral or Membranous Wall

1. The lateral wall separates the middle ear from the external acoustic meatus. 

It is formed:

a. Mainly by the tympanic membrane along with the tympanic ring and sulcus.

b. Partly by the squamous temporal bone, in the region of the epitympanic recess.

2. Near the tympanic notch, there are two small apertures.

a. The petrotympanic fissure lies in front of the upper

end of the bony rim. It lodges the anterior process of the malleus and transmits the tympanic branch of the maxillary artery.

b. The anterior canaliculus for the chordatympani nerve lies either in the fissure or just in front of it. 

The  nerve leaves the middle ear through this canaliculus to emerge at the base of the skull

Medial or Labyrinthine Wall

The medial wall separates the middle ear from the internal ear. It presents the following features.

1. The promontory is a rounded bulging produced by the first turn of the cochlea.

It is grooved by the tympanic plexus.

2. The fenestra vestibuli is an oval opening posterosuperior to the promontory. 

It leads into the vestibule of the internal ear and is closed by the foot-plate of the stapes.

3. The prominence of the facial canal runs backwards just above the fenestra vestibuli, to reach the lower margin of the aditus. 

The canal then descends behind the posterior wall to end at the stylomastoid foramen.

4. The fenestra cochleae is a round opening at the bottom of a depression posteroinferior to the promontory.

It opens into the scala tympani of the cochlea, and is closed by the secondary tympanic membrane.

5. The sinus tympani is a depression behind the promontory, opposite the ampulla of the posterior semicircular canal.

6. The processus cochleariformis.

7. Prominence of lateral semicircular canal above the facial canal.

Arterial Supply

The main arteries of the middle ear are as follows.

1. The anterior tympanic branch of the maxillary artery

which enters the middle ear through the petrotympanic fissure.

2. The posterior tympanic branch of the stylomastoid branch of the posterior auricular artery which enters through the stylomastoid foramen.

Venous Drainage

Veins from the middle ear drain into the superior petrosal sinus and the pterygoid plexus of the veins.

Lymphatic Drainage

Lymphatics pass to the preauricular and retropharyngeal lymph nodes.

Nerve Supply

The nerve supply is derived from the tympanic plexus which lies over the promontory. The plexus is formed by the following.

1. The tympanic branch of the glossopharyngeal nerve.

Its fibres are distributed to the mucous membrane of the middle ear, the auditory tube, the mastoid antrum and air cells. It also gives off the lesser petrosal nerve.

2. The superior and inferior caroticotympanic nerves

arise from the sympathetic plexus around the internal carotid artery. These fibres are vasomotor to the mucous membrane.

FUNCTIONS OF THE MIDDLE EAR

1. It transmits sound waves from the external ear to the internal ear through the chain of ear ossicles, and

thus transforms the air-borne vibrations from the tympanic membrane to liquid-borne vibrations in the

internal ear.

2. The intensity of the sound waves is increased ten times by the ossicles. It may be noted that the

frequency of sound does not change.

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Middle Ear | Situation | Dimensions | Parts | Boundaries | Contents