[Illustration:  Fig. 140.—­Ear-Bones. (Anterior View.)

  1, malleus, or hammer;
  2, incus, or anvil;
  3, stapes, or stirrup.
]

345.  The Internal Ear.  This forms one of the most delicate and complex pieces of mechanism in the whole body.  It is that portion of the organ which receives the impression of sound, and carries it directly to the seat of consciousness in the brain.  We are then able to say that we hear.

The internal ear, or bony labyrinth, consists of three distinct parts, or variously shaped chambers, hollowed out in the temporal bone,—­the vestibule, the semicircular canals, and the cochlea, or snail’s shell.

[Illustration:  Fig. 141.—­A Cast of the External Auditory Canal.  (Posterior view)]

The vestibule is the common cavity with which all the other portions of the labyrinth connect.  It is an oval-shaped chamber, about 1/3 of an inch in diameter, occupying the middle part of the internal ear.  It is on the inner side of the oval window, which was closed, as we have seen, by the stirrup bone.  From one side of this vestibule, or central hall, the three semicircular canals pass off, and from the other side, the cochlea.

The three semicircular canals, so called from their shape, are simply bony tubes about 1/20 of an inch in width, making a curve of about 1/4 of an inch in diameter.  They pass out from the vestibule, and after bending around somewhat like a hoop, they return again to the vestibule.  Each bony canal contains within it a membranous canal, at the end of which it is dilated to form an ampulla.

Experiment 157. To vibrate the tympanic membrane and the little ear-bones. Shut the mouth, and pinch the nose tightly.  Try to force air through the nose.  The air dilates the Eustachian tube, and is forced into the ear-drum.  The distinct crackle, or clicking sound, is due to the movement of the ear-bones and the tympanic membrane.

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.