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ANATOMY OF THE EAR                              261

duct from the saccule ending in the endolymphatic sac {see Fig. 138) which
lies between the layers of dura mater on the posterior surface of the petrous
temporal bone midway between the internal acoustic meatus and the lateral
sinus (see Fig* 136).

The membranous cochlea. This is sometimes called the ductus cochlearis or
scala media, and is a blind tube, triangular in section, coiled round a central
bony pillar called the modiolus. The floor of the tube, which is longer at the
apex of the modiolus than at the base, rests on the basilar membrane on the
inner part of which lies a mound of neuroepithelium called the organ of
Corti. In the lateral wall or short side of the tube there is a layer of vascular
epithelium known as the stria vascularis which is concerned with formation of
the endolymph. The sloping roof and the third wall of the tube are formed
by the vestibular (Reissner's) membrane completing the separation of the
scala media from the scala vestibuli. The narrowest part of the membranous
cochlea which lies within the vestibule is connected to the saccule by a fine
duct (ductus reuniens) {see Fig. 138).

The fluid system of the labyrinth is divided into two streams, endolymph
and perilymph. The belief that endolymph, contained in the membranous
labyrinth, was simply derived from the cells of the striae vascularis and
absorbed from the saccus endolymphaticus has been questioned. Recent
investigation suggests that separate endolymph circulations are present in
the pars superior and the pars inferior of the membranous labyrinth but the
exact mechanisms of production and disposal remain undecided.

Perilymph, which occupies the perilymphatic space of the bony labyrinth,
though similar to cerebrospinal fluid, contains more protein and much less
chloride. Whilst not yet proved it is believed that perilymph is derived from
endolymph by diffusion through Reissner's membrane or the perilymphatic
blood vessels, and is not, as previously thought, part of a continuous system
with, and derived from, cerebrospinal fluid in the posterior cranial fossa.

Organ of Corti. The organ of Corti, situated on the basilar membrane and
consisting of a complex arrangement of supporting and hair cells, is the area
of commencement of the sensory transmission of sound to the auditory
centres. In fulfilling this function the basilar membrane and the tectorial
membrane, which is in contact with the hair cells of the organ of Corti, are
necessary components. In its ascent from the basal coil of the scala media to
the apical coil structural changes are found in the organ consisting of an
increase in the width of the basilar membrane and hi its fibrous tissue content.
The tectorial membrane becomes larger, the tunnel of Corti enclosed by inner
and outer rods increases in height and width and the nerve supply to the hair
cells decreases.

Vessels of internal ear. The arteries are the labyrinthine arising from the
basilar or anterior inferior cerebellar artery and the stylomastoid, a branch
of the posterior auricular or occipital artery. The veins unite to form the
labyrinthine vein which opens into the inferior petrosal sinus or the sigmoid
sinus. Small veins pass via the aqueducts of the vestibule and cochlea to the
superior and inferior petrosal sinuses respectively.

Vestibulocochlear nerve. The vestibulocochlear (acoustic) nerve is formed
by cochlear and vestibular parts in the internal acoustic meatus from which
it emerges on the lateral side of the sensory root of the facial nerve and
enters the brain stem between the pons and the medulla. The cochlear part is