Surgical anatomy of the ear
The external
ear consists of the pinna and the ear canal. The pinna is made of yellow
elastic cartilage covered by tightly adherent skin. The external and middle ear
develop from the first two branchial arches. The external ear canal is 3 cm in
length, the outer two-thirds is cartilage and the inner third is bony. The skin
on the lateral surface of the tympanic membrane and the inner two-thirds of
the ear canal is highly specialised. It does not simply shed like the skin
from the rest of the body. It migrates outwards from the tympanic membrane and
along the ear canal. As a result of this migration most people’s ears are
self-cleaning. Disorders of skin migration can result in ear disease (e.g.
cholesteatoma). The external canal is richly innervated and the skin is tightly
bound down to the perichondrium so that oedema in this region results in severe
pain.
The
lymphatics of the external ear drain to the retro auricular, parotid,
retropharyngeal and deep upper cervical lymph nodes.
The
middle ear contains the ossicles. Laterally it is bounded by the
tympanic membrane, medially by the cochlea, anteriorly by the eustachian tube
and posteriorly it communicates with the mastoid air cells (Fig.
40.1).
Entwined in this tiny space is the facial nerve which pursues a tortuous course
through the middle ear and exits the skull base at the stylomastoid foramen.
Knowledge of the anatomy of the
The
tympanic membrane has three layers: an inner mucosal layer, a dense
fibrous middle layer and the outer stratified squamous epithelium (skin). The
upper portion that lies above the lateral process of the malleus is called the
pars flaccida. The lower portion, making up the majority of the drum, is called
the pars tensa (Fig. 40.2).
The
tympanic membrane and ossicles act as a transformer system converting vibrations
in the air to vibrations within the fluid-filled inner ear (perilymph).
The
inner ear comprises the cochlea and vestibular labyrinth (saccule,
utricle and semicircular canals). These structures are embedded in dense bone
called the otic capsule.
There
are approximately 15 000 hair cells in the human cochlea. They are arranged in
rows of inner and outer hair cells. The inner hair cells act as
mechanicoelectric transducers, converting the acoustic signal into an electric
impulse. The outer hair cells contain contractile proteins and
Each
inner hair cell responds to a particular frequency and when stimulated it
depolarises and passes an impulse to the cochlea nuclei in the brainstem.
The
vestibular labyrinth consists of the semicircular canals, the utricle and
saccule, and their central connections. The three semicircular canals are
arranged in the three planes of space at right angles to each other. As in the
auditory system, hair cells are present. In the lateral canals the hair cells
are embedded in a gelatinous cupula, and shearing forces, caused by angular
movements of the head, produce hair cell movements and generate action
potentials. In the utricle and saccule the hair cells are embedded in an
otoconial membrane which contains particles of calcium carbonate. These
respond to changes in linear acceleration and the pull of gravity.
Impulses
are carried centrally by the vestibular nerve, and connections are made to the
spinal cord, cerebellum and external ocular muscles.
The
sensory nerve supply of the ear is complex. The external ear is
supplied by the auriculotemporal branch of the trigeminal nerve (V), and this
supplies most of the anterior half of the pinna and the external auditory meatus.
The greater auricular nerve (C2,3), together with branches of the lesser
occipital nerve (C2), supply the posterior part of the pinna. The VIIth, IXth
and Xth cranial nerves also supply small sensory branches to the external ear;
this explains why the vesicles of herpes zoster affecting the VIIth nerve appear
in the concha (see Fig. 40.29 later). The middle ear is supplied by the
glossopharyngeal nerve (IX).
This
complicated and rich sensory innervation means that referred otalgia is common
and may originate from the normal area of distribution of any of the above
nerves. A classic example is the referred otalgia caused by a malignancy in the
pyriform fossa of the pharynx or a cancer of the larynx.
Anatomy of the ear
•
Middle ear is intimately related to the cranial cavity
•
The Vllth nerve has a tortuous course through the ear