The membranous labyrinth consists of: (1) the duct of the cochlea which lies in the bony cochlea; (2) the utricle and saccule—two small membranous sacs which lie in the vestibule; and (3) three mem- branous tubes—the semicircular ducts—which lie
Posterior semicircular duct Lateral
semicircular duct Anterior
semicircular duct
Duct of cochlea
Saccule Ductus reuniens
Endolymphatic duct
Endolymphatic sac Utricle
Fig. 13.14 Diagrammatic representation of the right membranous labyrinth.
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Organs of hearing and equilibrium
number of concretions of calcium salts—the stato
conia. The macula of the utricle lies in a horizontal plane, and that of the saccule lies in a vertical plane.
They record the direction of the gravitational field relative to the head. Both maculae are innervated by the vestibular part of the vestibulocochlear nerve.
See Clinical Applications 13.1, 13.2, 13.3, and 13.4 for the practical implications of the anatomy discussed in this chapter.
The utricle occupies a depression in the sup- erior wall of the vestibule, and the smaller saccule lies antero-inferior to it [Fig. 13.13B]. The endolym- phatic duct unites the utricle and saccule and passes through the aqueduct of the vestibule to end as a dilated endolymphatic sac, external to the dura mater. The utricle and saccule each have an area of thickened epithelium in their walls known as the macula. The macula has hair cells on which are a
CLINICAL APPLICATION 13.1 Tympanic membrane perforation and myringotomy Perforation of the tympanic membrane can occur as a result of
trauma or even spontaneously, when there is infection in the middle ear cavity (otitis media). Most often, these perforations heal spontaneously. However, large perforations may require repair. The temporalis fascia is often used as graft for repair of tympanic membrane perforations.
In some instances, it may be necessary to surgically incise the tympanic membrane (myringotomy) to relieve pres- sure built up within the middle ear cavity.
Study question 1: name the structure a surgeon must be careful to avoid during incision of the tympanic membrane.
What precaution should be taken to avoid this? (Answer: the chorda tympani. The surgeon should stay away from the up- per third of the tympanic membrane to avoid injury to the chorda tympani nerve.)
CLINICAL APPLICATION 13.2 Otitis media Otitis media is inflammation within the middle ear cavity. It
could be the result of an acute infection and could resolve spontaneously or with antibiotics. Acute otitis media is com- mon in children, often beginning as an upper respiratory tract infection with a blocked nose. It presents with fever and severe earache, often leading to rupture of the tympan- ic membrane and ear discharge.
Study question 1: explain the anatomical basis for up- per respiratory tract infections leading on to otitis media.
(Answer: the mucosa of the nasopharynx is continuous with that of the middle ear cavity through the auditory
tube. Infections can spread from the nasopharynx to the middle ear cavity through the tube. Oedema and blockage of the auditory tube can further increase the risk of infection.) Chronic otitis media refers to long-term damage of the middle ear mucosa due to repeated in- fection and inflammation. It is invariably associated with a perforated tympanic membrane, repeated episodes of ear discharge, and conductive hearing loss. An invasive, non-cancerous epithelial growth can occur within the middle ear— cholesteatoma—and cause extensive bone damage.
CLINICAL APPLICATION 13.3 Mastoiditis Mastoiditis is inflammation of the mucosa lining the mas-
toid antrum and air cells. The mucosa of the middle ear is continuous with the mucosa of the mastoid antrum, and infections can spread from one cavity to the other. Surgery for chronic otitis media with mastoiditis aims to remove dis- eased tissue and repair the tympanic membrane to restore hearing. The common approach to the mastoid antrum is through its lateral wall—the suprameatal triangle.
Study question 1: discuss the important relations of the mastoid antrum. (Answer: the vertical course of the facial nerve lies along the anterior wall of the mastoid antrum.
The cranial cavity lies just above the roof. The sigmoid venous sinus lies posterior to the antrum at a variable distance, depending on the pneumatization of the mas- toid process.)
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The internal ear
CLINICAL APPLICATION 13.4 Facial nerve lesions Clinically, it is possible to differentiate injuries to the facial
nerve at different sites along its course. (1) When the facial nerve is affected at, or distal to, the stylomastoid foramen, the patient will show only signs of paralysis of the facial muscles on that side. (2) When the facial nerve is affected in the brain, in the internal acoustic meatus, or at the level of the genicular ganglion, in addition to paralysis of facial muscles, the patient will also have loss of taste on the anterior two-thirds of the
corresponding half of the tongue (due to destruction of fibres in the chorda tympani) and hyperacusis (due to destruction of the nerve to the stapedius). Secretions of the subman- dibular and sublingual glands and lacrimal, nasal, palatine, and oral glands will be diminished (due to damage to the secretomotor fibres in the greater petrosal and chorda tympani nerves). An injury in the internal acoustic meatus is also likely to involve the vestibulocochlear nerve.