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250                                           THE EAR

The middle ear or tympanic cavity lies between the tympanic membrane
laterally and the cochlea medially. Its upper part extending above the
tympanic membrane is known as the epitympanic recess or attic, and the
lower part extending below the level of the floor of the external auditory
meatus is referred to as the hypotympanum (Fig. 128).

The cavity may be described as a six-sided box, frequently likened in shape
to a match-box standing on end with its vertical length greater than its

Fig. 128. Vertical coronal section through left ear, viewed from behind. 1, Middle cranial fossa; 2,
Attic; 3, Lateral semicircular canal; 4, Posterior vertical semicircular canal; S, Facial nerve; 6, Vestibule;
7, Internal acoustic meatus with nerves; 8, Internal carotid artery; 9, Hypotympanum; 10, Medial
surface of drumhead; 11, Floor of bony meatus; 12, Junction of bony with cartilaginous meatus;
13, Incus.

breadth but narrow in depth, particularly in the central portion where the
basal turn of the cochlea forms a bulge on the medial wall. The roof of the
cavity is formed by a thin plate of bone (the tegmen tympani) from the
petrous part of the temporal bone which joins the squamous part. This plate
of bone also forms the roof of the mastoid antrum and separates the tympanic
cavity and antrum from the middle fossa of the skull. The floor which is also
thin separates the cavity from the bulb of the internal jugular vein which may
be exposed by bony deficiency. The tympanic branch of the glossopharyngeal
nerve enters the cavity through the floor.

The anterior watt in its lower portion is formed by a thin plate of bone
separating the cavity from the internal carotid artery. The upper portion has
two openings, the lower one being the auditory (Eustachian) tube and above
it the canal for the tensor tympani muscle.

The posterior wall is wider than the anterior wall and in its upper part the
aditus connects the epitympanic recess (attic) with the mastoid antrum.
Below the aditus a bony projection, the pyramid, gives exit to the tendon of
the stapedius muscle. Just above the pyramid the fossa incudis gives attach-
ment for the short process of the incus. The facial nerve bends downwards at