the extent of this power imagine an auditor in a large music
hall where a full band and chorus are performing. Here, there are
sounds mingled together of all varieties of pitch, loudness, and
quality; stringed instruments, wood instruments, brass instruments,
and voices, of many different kinds. And in addition to these there
may be all sorts of accidental and irregular sounds and noises, such
as the trampling and shuffling of feet, the hum of voices, the rustle
of dress, the creaking of doors, and many others. Now it must be
remembered that the only means the ear has of becoming aware of these
simultaneous sounds is by the condensations and rarefactions which
reach it; and yet when the sound wave meets the nerves, the nerves
single out each individual element, and convey to the mind of the
hearer, not only the tones and notes of every instrument in the
orchestra, but the character of every accidental noise; and almost as
distinctly as if each single tone or noise were heard alone."--POLE.

[Illustration: FIG. 194.--The ear.]

277. The Structure of the Ear. The external portion of the ear acts
as a funnel for catching sound waves and leading them into the canal,
where they strike upon the ear drum, or tympanic membrane, and throw
it into vibration. Unless the ear drum is very flexible there cannot
be perfect response to the sound waves which fall upon it; for this
reason, the glands of the canal secrete a wax which moistens the
membrane and keeps it flexible. Lying directly back of the tympanic
membrane is a cavity filled with air which enters by the Eustachian
tube; from the throat air enters the Eustachian tube, moves along it,
and passes into the ear cavity. The dull crackling noise noticed in
the ear when one swallows is due to the entrance and exit of air in
the tube. Several small bones stretch across the upper portion of the
cavity and make a bridge, so to speak, from the ear drum to the far
wall of the cavity. It is by means of these three bones that the
vibrations of the ear drum are transmitted to the inner wall of the
cavity. Behind the first cavity is a second cavity so complex and
irregular that it is called the labyrinth of the ear. This labyrinth
is filled with a fluid in which are spread out the delicate sensitive
fibers of the auditory nerves; and it is to these that the vibrations
must be transmitted.

Suppose a note of 800 vibrations per second is sung. Then 800 pulses
of air will reach the ear eac