The =membranous labyrinth= (Labyrinthus membranaceus) lies within, but
does not fill, the osseous labyrinth. It is attached to the latter by
delicate trabeculæ which traverse the perilymphatic space. It conforms
more or less closely to the bony labyrinth, but consists of four
divisions, since the vestibule contains two membranous sacs—the utricle
and saccule.
1. The =utricle= (Utriculus), the larger of the two sacs, lies in the
postero-superior part of the vestibule, largely in the recessus
ellipticus. It receives the openings of the membranous semicircular
canals, and the small =ductus utriculo-saccularis= leads from its lower
part to the ductus endolymphaticus.
2. The =saccule= (Sacculus) is situated in the recessus sphæricus of the
vestibule. From its lower part the =ductus reuniens= proceeds to open
into the ductus cochlearis, a little in front of the blind end of the
latter. A second narrow tube, the =ductus endolymphaticus=, passes from
the posterior part of the saccule, and is joined by the ductus
utriculo-saccularis; it then traverses the aquæductus vestibuli, and
terminates under the dura mater on the posterior part of the internal
surface of the petrous temporal bone in a dilated blind end, the saccus
endolymphaticus.
3. The =membranous semicircular canals= (Ductus semicirculares)
correspond in general to the osseous canals already described, but it
may be noted that while the ampullæ of the membranous canals nearly fill
those of the osseous canals, the other parts of the membranous canals
only occupy about one-fourth of the bony cavities.
4. The =cochlear duct= (Ductus cochlearis) is a spiral tube situated
within the cochlea. It begins by a blind end (Cæcum vestibulare) in the
cochlear recess of the vestibule, and ends by a second blind end (Cæcum
cupulare), which is attached to the cupola of the cochlea. The
vestibular part is connected with the saccule by the ductus reuniens.
The duct is triangular in cross-section, and it is usual to regard it as
having three walls. The vestibular wall or roof, which separates the
cochlear duct from the scala vestibuli, is formed by the very delicate
=membrana vestibularis= (of Reissner), which extends obliquely from the
lamina spiralis ossea to the outer wall of the cochlea. The tympanic
wall or floor intervenes between the cochlear duct and the scala
tympani; it is formed by the periosteum of the marginal part of the
lamina spiralis and the =membrana basilaris=, which stretches between
the free edge of the lamina and the outer wall of the cochlea. The outer
wall consists of the fibrous lining of the cochlea, which is greatly
thickened to form the =ligamentum spirale cochleæ=.
[Illustration: