=The Membranous Labyrinth.=--"The membranous vestibule, lying in the bony, consists of two sacs communicating by a narrow aperture. The posterior is called the _utriculus_, and into it the membranous semicircular canals open. The anterior, called the _sacculus_, communicates by a tube with the membranous cochlea. The membranous semicircular canals much resemble the bony, and each has [Illustration: FIG. 20.--A section through the cochlea in the line of its axis.] [Illustration: FIG. 21.--Section of one coil of the cochlea, magnified. _SV_, _scala vestibuli_; _R_, membrane of Reissner; _CC_, membranous cochlea (_scala media_); _lls_, _limbus laminæ spiralis_; _t_, tectorial membrane; _ST_, _scala tympani_; _lso_, spiral lamina; _Co_, rods of Corti; _b_, basilar membrane. ] an ampulla; in the ampulla one side of the membranous tube is closely adherent to its bony protector; at this point nerves enter the former. The relations of the membranous to the bony cochlea are more complicated. A section through this part of the auditory apparatus (Fig. 20) shows that its osseous portion consists of a tube wound two and a half times round a central bony axis, the _modiolus_. From the axis a shelf, the _lamina spiralis_, projects and partially subdivides the tube, extending farthest across in its lower coils. Attached to the outer edge of this bony plate is the membranous cochlea (_scala media_), a tube triangular in cross-section and attached by its base to the outer side of the bony cochlear spiral. The spiral lamina and the membranous cochlea thus subdivide the cavity of the bony tube (Fig. 21) into an upper portion, the _scala vestibuli_, _SV_, and a lower, the _scala tympani_, _ST_. Between these lie the lamina spiralis (_lso_) and the membranous cochlea (_CC_), the latter being bounded above by the membrane of Reissner (_R_) and below by the basilar membrane (_b_)."[20] The membranous cochlea does not extend to the tip of the bony cochlea; above its apex the scala vestibuli and scala tympani communicate. Both are filled with perilymph, so that when the stapes is pushed into the oval foramen, _o_, in Fig. 17, by the impact of an air-wave on the tympanic membrane, a wave of perilymph runs up the scala vestibuli to the top, where it turns into the scala tympani, down whose whorls it runs and pushes out the round foramen _r_, ruffling probably the membrane of Reissner and the basilar membrane on its way up and down. [Illustration: FIG. 22.--The rods of Corti. _A_, a pair of rods separated from the rest; _B_, a bit of the basilar membrane with several rods on it, showing how they cover in the _tunnel of Corti_; _i_, inner, and _e_, outer rods; _b_, basilar membrane; _r_, reticular membrane. ]