The ear consists of three parts- external ear, middle ear and internal ear
The External Ear
The external ear consists of the (i) auricle or pinna, (ii) external acoustic meatus and (iii) the tympanic membrane.
Auricle or Pinna
The entire pinna, except its lobule, and the outer part of external acoustic meatus is made up of a framework of a single piece of yellow elastic cartilage covered with skin.
The latter is closely adherent to the perichondrium on its lateral surface while it is slightly loose on the medial surface. Various elevations and depressions are seen on the lateral surface of pinna.
There is no cartilage between the tragus and crus of the helix. An incision made in this area will not cut through the cartilage and is used for endaural approach in surgery of the meatus or mastoid. Pinna is also the source of several graft materials for the surgeon.
Cartilage from the tragus, perichondrium from the tragus or concha and fat from the lobule are frequently used for reconstructive surgeries.
External Acoustic (Auditory) Meatus
External acoustic meatus extends from the bottom of the concha to the tympanic membrane and measures about 24 mm along its posterior wall. It is also called auditary canal or simply ear canal. The meatus is not a straight tube, its outer part is directed upwards, backwards and medially while its inner part is directed downwards, forwards and medially. Therefore, to see the tympanic membrane, the pinna has to be pulled upwards, backwards and laterally so as to bring the two parts in alignment.
The meatus is divided into two parts
It forms outer one-third (8 mm) of the meatus. Its cartilage is a continuation of the cartilage which forms the framework of the pinna. This cartilage is deficient at two places – the “fissures of Santorini” and through them, parotid or superficial mastoid infections can appear in the meatus or vice versa.
The skin covering the cartilaginous meatus is thick and contains ceruminous and pilosebaceous glands which secrete wax. Hair is only confined to the outer meatus and therefore furuncles (staphylococcal infection of hair follicles) are seen only in the outer third of the meatus.
It forms inner two-thirds (16 mm) of the meatus. Skin lining the bony meatus is thin and continuous over the tympanic membrane. It is devoid of hair and ceruminous glands. About 6 mm lateral to the tympanic membrane, the bony meatus presents a narrowing called the isthmus. Foreign bodies lodged medial to the isthmus get impacted and are difficult to remove.
Antero-inferior part of the deep meatus beyond the isthmus presents a recess called the anterior recess which acts as a cesspool for discharge and debris in cases of external and middle ear infections. Antero-inferior part of the bony meatus may present a deficiency (foramen of Huschke) in children up to the age of four or sometimes in adults, permitting infections and tumors to spread from external auditory canal to infra-temporal fossa and vice-versa.
Tympanic Membrane or the Drumhead
It forms the partition between the external acoustic meatus and the middle ear. It is obliquely set and as a result, its posterosuperior part is more lateral than its anteroinferior part. It is 9-10 mm tall, 8 mm wide and 0.1 mm thick. Tympanic membrane can be divided into two parts:
It forms most of the tympanic membrane. Its periphery is thickened to form a fibrocartilaginous ring called the annulus tympanicus which fits in the tympanic sulcus. The central part of pars tensa is tented inwards at the level of the tip of malleus and is called the umbo. A bright cone of light can be seen radiating from the tip of malleus to the periphery in the anteroinferior quadrant.
This is situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds (earlier called the malleolar folds). It is not so taut and may appear slightly pinkish. Various landmarks are seen on the lateral surface of the tympanic membrane.
Tympanic membrane consists of three layers:
- Outer epithelial layer which is continuous with the skin lining the meatus.
- Inner mucosal layer which is continuous with the mucosa of the middle ear.
- Middle fibrous layer which encloses the handle of malleus and has three types of fibres-the radia, circular and the parabolic.
Relations of external acoustic meatus are as follows:
- Superiorly: Middle cranial fossa
- Posteriorly: Mastoid air cells and the facial nerve
- Inferiorly: Parotid gland
- Anteriorly: Temporomandibular joint
The external ear is supplied by greater auricular nerve, lesser occipital nerve, auricular branch of the vagus nerve, auriculotemporal nerve and some fibers of the facial nerve.
Anterior wall and roof of the meatus and anterior part of the lateral surface of the tympanic membrane are supplied by auriculotemporal while posterior wall and floor of the meatus and posterior part of tympanic membrane are supplied by vagus nerve. Medial side of tympanic membrane receives sensory innervation through tympanic plexus (CN IX).
The Middle Ear
The middle ear together with the Eustachian tube, aditus, antrum, and mastoid air cells is called the middle ear cleft. It is lined by mucous membrane and filled with air.
The middle ear extends much beyond the limits of tympanic membrane which forms its lateral boundary and is sometimes divided into
- mesotympanum (lying opposite the pars tensa)
- epitympanum or the attic (lying above the pars tensa but medial to Shrapnell’s membrane and the bony lateral attic wall)
- hypotympanum (lying above the level of pars tensa). The portion of middle ear around the tympanic orifice of the Eustachian tube is sometimes called the protympanum.
Middle ear can be likened to a six-sided box with a roof, a floor, medial, lateral, anterior and posterior walls.
The roof is formed by a thin plate of bone called tegmen tympani. It also extends posteriorly to form the roof of the aditus and antrum. It separates the tympanic cavity from the middle cranial fossa.
The floor is also a thin plate of bone which separates the tympanic cavity from the jugular bulb. Sometimes it is congenitally deficient and the jugular bulb may then project into the middle ear, separated from the cavity by the mucosa only.
The anterior wall has a thin plate of bone which separates the cavity from the internal carotid artery. It also has two openings; the lower one for the Eustachian tube and the upper one for the canal of tensor tympani muscle.
The posterior wall lies close to the mastoid air cells. It presents a bony projection called pyramid through the summit of which appears the tendon of the stapedius muscle to get the attachment to the neck of stapes. Aditus, an opening through which attic communicates with the antrum, lies above the pyramid. Facial nerve runs in the posterior wall just behind the pyramid.
Facial recess or the posterior sinus is a depression. In the posterior wall lateral to the pyramid. It is bounded medially by the vertical part of VIIth nerve, laterally by the chorda tympani and above by the fossa incudis. Surgically facial recess is important as direct access can be made through this into the middle ear without disturbing posterior meatal wall (intact canal wall technique).
The medial wall is formed by the labyrinth. It presents a bulge called promontory which is due to the basal coil of cochlea; oval window into which is fixed the footplate of stapes; round window or the fenestra cochleae which is covered by the secondary tympanic membrane.
Above the oval window is the canal for facial nerve. Its bony covering may sometimes be congenitally dehiscent and the nerve may lie exposed making it very vulnerable to injuries or infection. Above the canal for facial nerve is the prominence of lateral semicircular canal. Just anterior to the oval window, the medial wall presents a hook-like projection called the processus cochlear informis.
The tendon of tensor tympani takes a turn here to get the attachment to the neck of the malleus. The cochleariform process also marks the level of the genu of the facial nerve. Medial to the pyramid is a deep recess called sinus tympani which is bounded by the subiculum below and the ponticulus above.
The lateral wall is formed largely by the tympanic membrane and to a lesser extent by the bony outer attic wall called the scutum. The tympanic membrane is semitransparent and forms a “window” into the middle ear.
It is a large, air-containing space in the upper part of mastoid and communicates with the attic through the aditus. Its roof is formed by the tegmen antri which separates it from the middle cranial fossa. The lateral wall of antrum is formed by a plate of bone which is on an average 1.5 cm thick in the adult. It is marked externally, on the surface of mastoid by suprameatal (MacEwen’s) triangle.
Aditus and Antrum
Aditus is an opening through which the attic communicates with the antrum. The bony prominence of the horizontal canal lies on its medial side while the fossa incudis, to which is attached the short process of incus, lies laterally. Facial nerve courses just below the aditus.
The internal ear or the labyrinth is the organ of hearing and balance. It consists of a bony and a membranous labyrinth. The membranous labyrinth is filled with a clear fluid called endolymph while the space between membranous and bony labyrinth is filled with perilymph.
It consists of three parts: the vestibule, the semicircular canals, and the cochlea.
Vestibule is the central chamber of the labyrinth. In its lateral wall lies the oval window. The inside of its medial wall presents two recesses, a spherical recess, which lodges the saccule and an elliptical recess which lodges the urile. Below the elliptical recess is the opening of aqueduct of vestibule through which passes the endolymphatic duct. In the posterosuperior part of the vestibule are the five openings of semicircular canals.
Semicircular canals are three in number: lateral, posterior and superior. They lie in planes at right angles to one another. Each canal has an ampullated end which opens independently into the vestibule and a non-ampullated end. The non-ampullated ends of posterior and superior canals unite to form a common channel called the crus commune. Thus the three canals open into the vestibule by five openings.
The bony cochlea is a coiled tube making two and three quarter turns around a central pyramid of bone called the modiolus. The base of modiolus is directed towards internal acoustic meatus and transmits vessels and nerves to the cochlea. Around the modiolus and winding spirally like the thread of a screw, is a thin plate of bone called osseous spiral lamina. It divides the bony cochlea incompletely and gives attachment to the basilar membrane. The bony bulge in the medial wall of the middle ear-the promontory is due to the basal coil of the cochlea. The bony cochlea contains three compartments:
- Scala vestibule
- Scala tympani
- Scala media or the membranous cochlea
The scala vestibule and scala tympani are filled with perilymph and communicate with each other at the apex of cochlea through an opening called helicotrema. Scala vestibule is closed by the footplate of stapes which separates it from the air-filled middle ear. The scala tympani is closed by the secondary tympanic membrane; it is also connected with the subarachnoid space through the aqueduct of cochlea.
It consists of the cochlear duct, the utricle, and saccule, the three semicircular ducts and sac.
Cochlear duct is also called membranous cochlea or the scala media. It is a blind coiled tube. It appears triangular in cross-section and its three walls are formed by:
- The basilar membrane, which supports the organ of corti
- The Reissner’s membrane which separates it from the scala vestibule
- The stria vascularis, which contains vascular epithelium and is concerned with the secretion of endolymph.
Cochlear duct is connected to the saccule by ductus reuniens. The length of the basilar membrane increases as we proceed from the basal coil to the apical coil. It is for this reason that higher frequencies of sound are heard at the basal coil while lower ones are heard at the apical coil.
The utricle lies in the posterior part of the bony vestibule. It receives the five openings of the three semicircular ducts. It receives the five openings of the three semicircular ducts. It is also connected to the saccule through utriculosaccular duct. The sensory epithelium of the utricle is called the macula and is concerned with linear acceleration and deceleration.
The saccule also lies in the bony vestibule, anterior to the utricle and opposite the stapes footplate. Its sensory epithelium is also called the macula. Its exact function is not known. It probably also responds to linear acceleration and deceleration. In Meniere’s disease, the distended saccule lies against the stapes footplate and can be surgically decompressed by perforating the footplate.
Semicircular ducts are three in number and correspond exactly to the three bony canals. They open in the utricle. The ampullated end of each duct contains a thickened ridge of neuro-epithelium called crista ampullaris.
Endolymphatic duct is formed by the union of two ducts, one each from the saccule and the utricle. It passes through the vestibular aqueduct. Its terminal part is dilated to form endolymphatic sc which lies between the two layers of dura on the posterior surface of the petrous bone. Endolymphatic sac is surgically important. It is exposed for drainage or shunt operation in Meniere’s disease.
Blood Supply of Labyrinth
The entire labyrinth receives its arterial supply from the internal auditory (labyrinthine) artery which is a branch of anterior inferior cerebellar artery or sometimes the basilar artery. The internal auditory artery divides into anterior vestibular and the common cochlear arteries. The latter further divides into cochlear and posterior vestibular arteries. There is no collateral circulation in the cochlea.