The nervous mechanisms which are most important for the appreciation of tones are those situated in the cochlea.
The nerve endings found in the membranous sacs in the vestibule are connected with peculiar epitheloid cells, to which are attached fine bristle-like processes. These processes lie in the endolymph, and are related to calcareous masses called otoliths. Waves in this endolymph possibly bring the otoliths into collision with the hairs, and thus give a stimulus to the nerve endings. Noises may be heard from this, but no impressions of tone can be appreciated. The use of the nerves going to the other parts of the labyrinth - ampulla of the semicircular canals - is doubtful, and probably not immediately connected with hearing.* The coils of the cochlea are, throughout their entire length, partially divided by a bony shelf projecting from the central axis into the spiral cavity. This is called the osseous spiral lamina. In the fresh state the separation of the spiral canal into an upper (vestibular) and a lower (tympanic) coil is completed by a membranous partition, which stretches from the bony spiral lamina to the opposite side of the spiral canal. This is called the membranous spiral lamina, and forms the base upon which the special nerve endings of the organ of hearing are placed. An extremely delicate membrane called the membrane of Reissner stretches from the upper side of the spiral partition obliquely upward to the outer wall of the spiral cavity, so as to form a canal and cover the special organ, shutting off a portion of the vestibular coil which lies over the membranous spiral lamina. The canal of the cochlea thus formed is triangular in section. Its floor is made up chiefly of the membranous spiral lamina, particularly the part called the basilar membrane, while the oblique roof is composed of only the thin membrane of Reissner. The canal follows the turns of the cochlea, lying between the vestibular coil and that leading to the tympanum, and is filled with a fluid (endolymph) which is quite separate and distinct from that in the vestibular or tympanic coils (perilymph).
The cochlear division of the auditory nerve passes into little tunnels in the central bony column around which the coils of the cochlea turn, and gives off a series of spiral branches which run through the osseous spiral lamina to reach the membranous portion. A collection of ganglion cells connected with the radiating nerve fibres is found lying in the spiral canal of the osseous lamina. Passing through the bony spiral the nerves reach the basilar membrane, which, as before mentioned, forms a great part of the membranous spinal lamina, and upon which the organ of Corti is placed.
* Compare equilibration, in connection with which they will be described.
Fig. 239. Transverse section through the membranous canal of the cochlea. Striated zone of basilar membrane, a. Pectinate zone of the basilar membrane, b. Perforated zone of basilar membrane through which the nerves pass, c. Nerve fibres from spiral ganglion, d. Spiral ganglion, e. Limbus, f. Reissner's membrane, g. Tectorial membrane, h. Internal rod of Corti, i. External rod of Corti, m. Special cells receiving nerve terminals, o,p,p'. Epithelial cells covering the basilar membrane, q. Nerve fibres, s. Spiral ligament,t. (Cadiat).
The organ of Corti, placed upon the basilar membrane within the membranous canal of the cochlea, is made up of a series of peculiarly curved bars or fibres, called the rods of Corti, and some epitheloid cells provided with short, bristle-like processes. The rods of Corti are fixed by their broad bases upon the basilar membrane, and unite above in such a way that the outer and inner rods form a bow or arch. The spiral series of rods thus propped up against each other leave a small space or tunnel under them, which runs the entire length of the basilar membrane. Beside these rods of Corti are placed rows of cells of an epithelial type into which the nerve endings pass. From the upper surface of these cells, on a level with the apex or junction of the rods, a number of hair-like processes project. A delicate reticulated membrane lies over the rods and the cells, and seems to be lightly attached to their surface, while the hairs pass through its meshes.
The basilar membrane is made up of fibrous bands held together by a delicate membrane. The fibres pass transversely across the spiral canal of the cochlea, so as to subtend the bases of the outer and inner rods. The basilar membrane gradually becomes wider as it passes from the base to the summit of the cochlea. The length of the rods also increases toward the summit of the organ, their bases being more widely separated from one another and their point of junction nearer to the basilar membrane, this forming a lower and wider tunnel. The number of rods of Corti has been estimated at 6000 inner and 4500 outer.