[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 ampullae 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.

Every time the stirrup bone is pushed in and drawn out of the oval window, the watery fluid (the perilymph) in the vestibule and inner ear is set in motion more or less violently, according to the intensity of the sound.  The membranous labyrinth occupies the central portion of the vestibule and the passages leading from it.  When, therefore, the perilymph is shaken it communicates the impulse to the fluid (endolymph) contained in the inner membranous bag.  The endolymph and the tiny grains of ear-sand now perform their part in this marvelous and complex mechanism.  They are driven against the sides of the membranous bag, and so strike the ends of the nerves of hearing, which transmit the auditory impulses to the seat of sensation in the brain.

It is in the seat of sensation in the brain called the sensorium that the various auditory impulses received from different parts of the inner ear are fused into one, and interpreted as sounds.  It is the extent of the vibrations that determines the loudness of the sound; the number of them that determines the pitch.

Experiment 158.  Hold a ticking watch between the teeth, or touch the upper incisors with a vibrating tuning-fork; close both ears, and observe that the ticking or vibration is heard louder.  Unstop one ear, and observe that the ticking or vibration is heard loudest in the stopped ear.

  Experiment 159.  Hold a vibrating tuning-fork on the incisor teeth
  until you cannot hear it sounding.  Close one or both ears, and you will
  hear it.