OF HEARING.--The external ear, although curiously shaped, is
not the most important part of the organ whose function it is to take
cognizance of sounds. In the transmission of sound to the brain, the
vibrations of the air produced by the sonorous body are collected by the
external ear, and conducted through the auditory canal to the drum of
the ear, which is so arranged that it may be relaxed or tightened like
the head of an ordinary drum. That its motion may be free, the air
contained within the drum has free communication with the external air
by an open passage, called the Eustachian tube, leading to the back of
the mouth. This tube is sometimes obstructed by wax, when a degree of
deafness ensues. But when the obstruction is removed in the effort of
sneezing or otherwise, a crack or sudden noise is generally experienced,
accompanied usually with an immediate return of acute hearing.

The ear-drum performs a two-fold office; for while it aids in the
transmission of sound from without to the internal ear, it at the same
time modifies the intensity of sound. This softening of the sound is
effected by the relaxation of a muscle when sounds are so acute as to be
painful; but when listening to low sounds, the drum is rendered tense
by the contraction of this muscle, and the sounds become, by this means,
more audible. The vibrations made on the drum are transmitted by the
tympanum--an irregular bony cavity--to the internal ear, which is filled
with a watery fluid. In this fluid the filaments of the auditory nerve
terminate, which receive and transmit the sound to the brain.

The ear has the power of judging of the direction from which sound
comes, as is strikingly exemplified in the fact that when horses or
mules march in company at night, those in front direct their ears
forward, and those in the rear turn them backward, while those in the
center turn them laterally or across, the whole troop seeming to be
actuated by a feeling to watch the common safety. This is also
illustrated by four or six horse teams, and is a fact with which
coachmen are familiar. It is further illustrated by the dog, and many
other animals. The external ear of man is likewise furnished with
muscles; and savages are said to have the power of moving or directing
their ears at pleasure, like a horse, to catch sounds as they come from
different directions; but few men in civilized life retain this power.

The acuteness of this sense in men and animals