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.
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.
The Terminal Organs.—"The membranous cochlea contains certain solid structures seated on the basilar membrane and forming the organ of Corti. This contains the end-organs of the cochlear nerves. Lining the sulcus spiralis, a groove in the edge of the bony lamina spiralis, are cuboidal cells; on the inner margin of the basilar membrane they become columnar, and then are succeeded by a row which bear on their upper ends a set of short stiff hairs, and constitute the inner hair-cells, which are fixed below by a narrow apex to the basilar membrane; nerve-fibres enter them. To the inner hair-cells succeed the rods of Corti (Co, [Fig. 21]), which are represented highly magnified in [Fig. 22.] These rods are stiff and arranged side by side in two rows, leaned against one another by their upper ends so as to cover in a tunnel; they are known respectively as the inner and outer rods, the former being nearer the lamina spiralis. The inner rods are more numerous than the outer, the numbers being about 6000 and 4500 respectively. Attached to the external sides of the heads of the outer rods is the reticular membrane (r, [Fig. 22]), which is stiff and perforated by holes. External to the outer rods come four rows of outer hair-cells, connected like the inner row with nerve-fibres; their bristles project into the holes of the reticular membrane. Beyond the outer hair-cells is ordinary columnar epithelium, which passes gradually into cuboidal cells lining most of the membranous cochlea. From the upper lip of the sulcus spiralis projects the tectorial membrane (t, [Fig. 21]) which extends over the rods of Corti and the hair-cells."[21]
