The cochlea, or snail’s shell, is another chamber hollowed out in the solid bone. It is coiled on itself somewhat like a snail’s shell. There is a central pillar, around which winds a long spiral canal. One passage from the cochlea opens directly into the vestibule; the other leads to the chamber of the middle ear, and is separated from it by the little round window already described. The cochlea contains thousands of the most minute cords, known as the fibers or _organ of Corti_.[49] Under the microscope they present the appearance of the keyboard of a piano. These fibers appear to vibrate in sympathy with the countless shades of sounds which daily penetrate the ear. From the hair-like processes on these tightly stretched fibers, auditory impulses appear to be transmitted to the brain. The tubes and chambers of the inner ear enclose and protect a delicate membranous sac of exactly the same shape as themselves. Between the bony walls of the passages and the membranous bag inside is a thin, clear fluid, the _perilymph_. The membranous bag itself contains a similar fluid, the _endolymph_. In this fluid are found some minute crystals of lime like tiny particles of sand, called _otoliths_, or ear-stones. Every movement of the fluid itself throws these grains from side to side. Illustration: Fig. 142.—Bony internal Ear of Right Side. (Magnified; the upper figure of the natural size.) A, oval window (foramen ovale); B, C, D, semicircular canals; * represents the bulging part (ampulla) of each canal; E, F, G cochlea, H, round window (foramen rotundum). The auditory nerve, or nerve of hearing, passes to the inner ear, through a passage in the solid bone of the skull. Its minute filaments spread at last over the inner walls of the membranous labyrinth in two branches,—one going to the vestibule and the ampullæ at the ends of the semicircular canals, the other leading to the cochlea. 346. Mechanism of Hearing. Waves of sound reach the ear, and are directed by the concha to the external passage, at the end of which they reach the tympanic membrane. When the sound-waves beat upon this thin membrane, it is thrown into vibration, reproducing in its movements the character of the air-vibrations that have fallen upon it. Now the vibrations of the tympanic membrane are passed along the chain of bones attached to its inner surface and reach the stirrup bone. The stirrup now performs a to-and-fro movement at the oval window, passing the auditory impulse inwards to the internal ear.