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This membrane extends the length of the cochlear canals, and is stretched between a projecting shelf of bone on one side and the outer wall of the cochlea on the other. It is covered with a layer of epithelial cells, some of which have small, hair-like projections and are known as the hair cells.
The most important part of the inner ear is 13, the cochlear canal, in which the "hair-cells" are found, around which latter the final branches of the auditory nerve end. Above it is the scala vestibuli and below it the scala tympani, passages filled with fluid. The openings to these canals are closed with membrane.
Through the cochlear nerves we learn the height and timbre, through the vestibular nerves the intensity, of tones. The first structure of this highly elaborate organ is very simple in the embryo of man and all the other Craniotes; it is a pit-like depression in the skin. This sinks into a sort of pit, and severs from the epidermis, just as the lens of the eye does.
It is estimated that there are no less than 3000 of these arches in the human ear, placed side by side in a continuous series along the whole length of the basilar membrane. The fact that these hair-cells are connected with the fibers of the cochlear division of the auditory nerve suggests that they must play an important part in auditory sensation.
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