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Full text of "Diseases Of The Nose Throat And Ear"

SECTION IV
THE EAR

G. D. McDowall and K. McLay

CHAPTER 46
ANATOMY OF THE EAR

Anatomically and clinically the ear is divided into three parts—the external
ear, the middle ear and the internal ear.

THE EXTERNAL EAR

The external ear consists of the auricle and the external acoustic meatus. The
auricle has two surfaces, lateral and medial; the parts of the lateral surface
are shown in Fig. 125. The form of the auricle is derived from a plate of
yellow fibrocartilage, absent in the lobule which is composed of fat and fibro-
areolar tissue. The skin on the lateral surface is closely adherent to the
perichondrium which predisposes to haematoma formation following injury
to this surface. Attachment of the auricle to the side of the head is by liga-
ments and muscles, the latter being rudimentary and largely functionless in
man. They are supplied by the facial nerve. The lymphatics of the auricle and
external meatus drain anteriorly into the pre-auricular glands, inferiorly into
superficial cervical glands along the external jugular vein and posteriorly into
the group of glands overlying the mastoid process. The posterior glands also
drain adjacent areas of the scalp, infection of which may produce swelling and
tenderness over the mastoid area.

The external acoustic meatus in the adult measures about J4jnm from the
introitus to the tympanic membrane but varies with growth and inlndividuals.
Since the tympanic membrane at the inner end of the meatus is obliquely
placed the anterior and inferior walls are longer than the posterior and
superior walls, and at the junction of the inferior wall with the tympanic
membrane a depression is formed, the inferior meatal recess, which may
contain infected debris.

The meatus is composed of two parts, an outer or lateral third having a
cartilaginous skeleton continuous with that of the auricle, and an inner or
medial two-thirds having a bony skeleton (Fig. 126). The general direction
of the cartilaginous meatus is medially, upwards and backwards whilst that
of the bony meatus is medially, slightly downwards and forwards. There are

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