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254                                         THE EAR

the superior petrosal sinus. The lymphatics mainly drain into the retro-
pharyngeal and parotid lymph glands.

The sensory nerve supply 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. Branches
pass to the greater superficial petrosal nerve and the otic ganglion via the
lesser superficial petrosal nerve from which secretomotor fibres are relayed by
the auriculotemporal nerve to the parotid gland.

The facial nerve after crossing the cerebellopontine angle where it is closely
related to the acoustic nerve 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 tympani 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 infra-
temporal fossa.

The auditory (Eustachian, pharyngotympanic) tube (Fig. 131) 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 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 (Fig. 133). The lateral wall
is membranous. In the resting state the lateral and medial walls lie in

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 con-
traction 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 the tensor tympani muscle and below it lies the jugular fossa.

The Eustachian tube is lined by columnar ciliated epithelium. The sub-
mucous tissue of the cartilaginous part contains numerous mucous glands.
The blood supply of the tube is from the ascending pharyngeal and middle
meningeal arteries and from the artery of the pterygoid canal. Veins drain
into the pterygoid plexus. The nerves are derived from the glossopharyngeal