The membranous labyrinth. The three membranous semicircular ducts
occupying, but not filling, the lumen of the bony canals communicate with
the utricle by five openings, the superior and posterior vertical ducts having
a common opening (crus commune) at their non-ampullated ends (see Fig.
138). The three ducts lie in the three planes of space and each duct is dilated
at one end to form an ampulla which contains a ridge of neuro-epithelium
termed the crista ampullaris (Fig. 144). The hair cells of the crista have long
Fig 143. Horizontal section through the cochlea showing: 1, Scala vestibuli; 2, Cochlear duct
(membranous labyrinth); 3, Scala tympani; 4, Cochlear nerve in internal acoustic meatus; 5, Spiral
ganglion in modiolus; 6, Helicotrema.
Fig. 144- Horizontal section through lateral semicircular canal (duct). 1, Perilymphatic space; 2, Endo-
lymphatic space; 3, Crista covered by cupula; 4, Vestibular nerve to crista; 5, Internal acoustic meatus.
filaments which project into a mass of gelatinous material called the cupula.
Movement of endolymph in the ducts bends the cupula and hair cells which
are supplied by the terminal fibres of the vestibular nerve.
The utricle, occupying the recessus ellipticus of the vestibule, also contains
a similar area of neuro-epithelium on the superior side of its lower wall. The
neuro-epithelium of the saccule, lying in the recessus sphericus, is placed on
the anteromedial wall at right angles to the plane of the sensory epithelium
of the utricle. In the utricle and saccule the neuro-epithelium is termed the
macula and from it hair cells are in contact with a membrane containing
otoliths or ear stones. From the utricle a small duct joins the endolymphatic