Incudis in the aditus, and the annular ligament attaches the footplate of the

stapes to the margins of the oval window. The folds of mucous membrane around

the ossicles and ligaments form spaces or pouches in which local suppuration may

occur.

Blood Vessels

The cavity is well supplied by:

- anterior tympanic artery (maxillary a.- E.C.A.),

- stylomastoid artery (posterior auricular a.- E.C.A.),

- middle meningeal artery (maxillary a. – E.C.A.),

- inferior tympanic artery (ascending pharyngeal a.- E.C.A.),

- caroto-tympanic branches of the internal carotid artery.

The veins drain into the pterygoid plexus, bulb of the internal jugular vein and

the superior petrosal sinus. The lymphatics mainly drain into the

retropharyngeal and parotid lymph glands.

Nerve Supply

This is through the tympanic plexus which lies on the promontory and is formed

by the tympanic branch of the glossopharyngeal nerve and the superior and

Inferior tympanic branches of the sympathetic plexus of the internal carotid

artery. The plexus supplies the lining mucosa of the tympanic cavity, the

mastoid air cells and the Eustachian tube.

Facial Nerve

After crossing the cerebellopontine angle where it is closely related to the

acoustic nerve it enters the temporal bone at the internal auditory meatus. It

passes laterally, curving slightly upwards over the labyrinth until it reaches

the anterior part of the medial wall of the tympanic cavity, close to the roof,

just behind or medial to the processus cochleariformis. Here it bends backwards

at right angles, where the geniculate ganglion is situated and passes almost

horizontally, enclosed in the Fallopian canal, above the oval window and below

the lateral semicircular canal. When it reaches the aditus it turns downwards

behind the pyramid and continues almost vertically until it emerges at the

stylomastoid foramen. The nerve to the stapedius muscle is given off close to

the pyramid. The chorda tympany nerve leaves the descending part of the facial

nerve and enters the tympanic cavity which it crosses, enclosed in a fold of

mucosa, to pass between the handle of the malleus and the long process of the

incus. It leaves the cavity through the medial end of the petrotympanic fissure

to join the lingual nerve in the infratemporal fossa.

The Eustachian Tube

The Eustachian tube connects the tympanic cavity with the nasopharynx and in the

adult is about 36 mm in length. From its pharyngeal end it runs upwards,

laterally and backwards. In infants the tube is shorter and wider and its course

is more horizontal than in adults. The tube has two parts: a pharyngeal

membrano-cartilaginous part which forms two-thirds of its length, and a tympanic

bony portion. The upper and medial walls of the pharyngeal portion of the tube

are formed by a plate of cartilage, hook-shaped in vertical coronal sections.

The lateral wall is membranous. In the resting state the lateral and medial

walls lie in apposition.

The fibres of origin of the tensor palati muscle are attached to the lateral

wall of the tube so that contraction of this muscle on swallowing or yawning

opens the tube, and thus equality of air pressure is maintained on both sides of

the tympanic membrane. The levator palati muscle runs below the floor of the

cartilaginous part of the tube and it is generally considered that contraction

of this muscle opens the tube.

The bony portion of the tube lies between the internal carotid artery medially

and the temporomandibular joint laterally. Above the tube is the bony canal for

tensor tympany muscle and below it lies the jugular fossa.

The Eustachian tube is lined by columnar ciliated epithelium. The submucous

tissue of the cartilaginous part contains numerous mucous glands. The blood

supply of the tube is from the ascending pharyngeal artery (E.C.A.) and middle

meningeal artery (maxillary artery – E.C.A.) and from the artery of the

pterygoid canal. Veins drain into the pterygoid plexus. The nerves are derived

from the glossopharyngeal nerve through the tympanic plexus and the maxillary

division of the trigeminal nerve through the pharyngeal branch of the

pterygopalatine ganglion.

Mastoid process

It lies below and behind the squamous, and behind and lateral to the petrous

portions. The mastoid process is absent at birth and in the infant the

stylomastoid foramen lies close to the surface. For this reason a mastoid

incision in young children should not be extended in a downward direction,

otherwise the facial nerve may be cut just after emerging from its foramen. The

mastoid or tympanic antrum which is present from birth communicates anteriorly

via the aditus with the epitympanic recess, and behind and below with the

mastoid air cells. In the infant it lies at a higher level than in the adult and

is superficial, being covered only by a thin plate of bone. The roof of the

tympanic antrum is the backward extension of the tegmen tympani. The floor and

medial wall are formed by the petromastoid bone, the medial wall being related

to the bony labyrinth .

The mastoid-cell system shows a considerable normal variation both in size and

in number of cells. The are four types of mastoid process : pneumatized,

sclerotic, diploetic and mixt. Where the cell system is extensive the mastoid is

referred to as well pneumatized, or cellular, while one composed mainly of

sclerotic bone is termed an acellular, or dense, mastoid. The cells are lined by

very thin, flattened epithelium. In a well-pneumatized mastoid the cells can be

divided into groups and are named according to their location: zygomatic,

perisinus, retrofacial, peri-antrum, subdural, sino-dural angle, tip,

perylabyrinth.

THE INTERNAL EAR

The internal ear within the petrous part of the temporal bone consists of a

membranous labyrinth (Fig. 7) enclosed in a bony (osseous) labyrinth (Fig. 8,

9). The membranous labyrinth contains fluid known as endolymph, and the space

within the bony labyrinth between its walls and the membranous labyrinth

contains fluid known as perilymph. The membranous labyrinth contains vestibular

and auditory components. The vestibular element, which is connected to the

vestibular nerve, consists of the three semicircular canals, the utricle and the

saccule. The auditory labyrinth consists of the cochlear duct, or membranous

cochlea, and is connected to the auditory nerve. The endolymphatic sac, which

lies on the posterior surface of the temporal bone, is connected to the

membranous labyrinth by the endolymphatic duct which passes through the medial

wall of the bony labyrinth. The interior of the bony labyrinth is lined by

endosteum. The central part of the bony labyrinth is the vestibule in the

lateral wall of which lies the fenestra vestibuli (oval window), closed by the

footplate of the stapes and the annular ligament. The medial or inner wall of

the vestibule is directed towards the internal acoustic meatus and is perforated

by nerve endings. It contains two depressions - the spherical recess for the

saccule and the elliptical recess for the utricle. Below the elliptical recess

there is the small opening of the aqueduct of the vestibule which transmits the

endolymphatic duct. The posterior, superior and inferior walls of the vestibule

contain the five openings of the semicircular canals. The horizontal canal has

an opening at each end but the frontal and sagital vertical canals have a common

opening at their non-ampullated ends, the crus commune.

The bony cochlea, anterior to the vestibule, is a spiral tube wound two and a

half times round its central axis or modiolus. Its base is directed towards the

internal acoustic meatus and its apex towards the internal carotid artery. From

the modiolus, which contains the cochlear nerve, a hollow bony ledge (spiral

lamina) projects into the lumen of the tube transmitting the branches of the

nerve. This ledge commences in the vestibule just above the fenestra cochleae

(round window) and ends near the apex of the cochlea at the hamulus or hook. The

basilar membrane of the membranous cochlea is attached to the edge of the bony

spiral lamina, and the outer surface of the membranous cochlea is attached to

the inner wall of the bony cochlea thus dividing each coil of the bony cochlea

into three parts: (1) the upper part, the scala vestibuli, commences from an

opening in the vestibule below the spherical recess; (2) the lower part, the

scala tympany, begins at the fenestra cochleae (round window); and (3) the

membranous cochlea, or scala media, lies between the upper and lower parts (Fig.

13). It is separated from the scala vestibuli by a membrane (Reissner's

membrane) and contains the organ of Corti. At the apex of the cochlea the

membranous cochlea ends blindly and in this region, known as the helicotrema,

the scala vestibuli and scala tympani are continuous with one another.

The Membranous Labyrinth

The three membranous semicircular ducts occupying, but not filling, the lumen of

the bony canals communicate with the utricle by five openings, the frontal and

sagital ducts having a common opening (crus commune) at their non-ampullated

ends. The three ducts lie in the three planes of space, and each duct is dilated

at one end to form an ampulla which contains a ridge of neuro-epithelium termed

the crista ampullaris (Fig. 10). The hair cells of the crista have long

filaments which project into a mass of gelatinous material called the cupula

(Fig. 12). Movement of ehdolymph in the ducts bends the cupula and hair cells

which are supplied by the terminal fibres of the vestibular nerve.

The utricle, occupying the recessus ellipticus of the vestibule, contains a

similar area of neuro-epithelium lying in a horizontal plane. The

neuro-epithelium of the saccule, lying in a vertical plane, occupies the

recessus sphericus. In the utricle and saccule the neuro-epithelium is termed

the macula, and form it hair cells are in contact with a membrane containing

otoliths or ear stones (Fig. 11). From the utricle a small duct joins the

endolymphatic duct from the saccule ending in the endolymphatic sac which lies

between the layers of dura mater on the posterior surface of the petrous

temporal bone midway between the internal acoustic meatus and the lateral sinus.

The Membranous Cochlea

This is sometimes called the cochlear duct or scala media, and is a blind tube,

triangular in section, coiled round a central bony pillar called the modiolus

(Fig. 14). The floor of the tube is formed by the basilar membrane on the inner

part of which lies a mound of neuro-epithelium called the organ of Corti. In the

lateral wall of the tube there is a layer of vascular epithelium known as the

stria vascularis. The sloping roof and the third wall of the tube are formed by

the vestibular (Reissner's) membrane completing the separation of the scala

media from the scala vestibuli. The narrowest part of the membranous cochlea

which lies within the vestibule is connected to the saccule by a fine duct

(ductus reuniens).

The fluid system of the labyrinth is divided into two components, endolymph and

perilymph. Endolymph, which is contained in the membranous labyrinth, has a high

potassium and a low sodium concentration. It may be produced by stria vascularis

and absorbed by the endolymphatic sac, but this is by no means proven.

Perilymph, which occupies the perilymphatic space of the bony labyrinth, is

similar to cerebrospinal fluid and contains a high sodium and low potassium

concentration. Whilst not yet proved, it is believed that perilymph is derived

mainly from blood vessels, although some is derived from the cerebrospinal fluid