bration. This membrane could not be made to
vibrate by the more delicate of the sound waves if it were stretched over
a bone, or over some of the softer tissues, or over a liquid. Its
vibration is made possible by the presence of air on _both_ sides, and
this condition is supplied, on the inner side, by the middle ear. The
Eustachian tube, by providing for an _equality_ of pressure on the two
sides of the membrane, also aids in this purpose.

In the second place, the middle ear provides a means for _concentrating
the force of the sound waves_ as they pass from the membrana tympani to
the internal ear. This concentration is effected in the following manner:

1. The bridge of bones, being pivoted at one point to the walls of the
middle ear, forms a lever in which the malleus is the long arm, and the
incus and stapes the short arm, their ratio being about that of three to
two. This causes the incus to move through a shorter distance, but with
greater force than the end of the malleus.

2. The area of the membrana tympani is about twenty times as great as the
membrane of the internal ear which is acted upon by the stapes. The force
from the larger surface is, therefore, concentrated by the bridge of bones
upon the smaller surface. By the combination of these two devices, the
waves striking upon the membrane of the internal ear are rendered some
thirty times more effective than are the same waves entering the auditory
canal.

*The Internal Ear*, or labyrinth, occupies a series of irregular channels
in the petrous process of the temporal bone.(121) It is very complicated
in structure, and at the same time is very small. Its greatest length is
not more than three fourths of an inch and its greatest diameter not more
than one half of an inch. It is filled with a liquid which at one place is
called the _perilymph_, and at another place the _endolymph_. It is a
double organ, being made up of an outer portion which lies next to the
bone, and which surrounds an inner portion of the same general form. The
outer portion is surrounded by a membrane which serves as periosteum to
the bone and, at the same time, holds the liquid belonging to this part,
called the perilymph. The inner portion, called the _membranous
labyrinth_, consists essentially of a closed membranous sac, which is
filled with the endolymph. The auditory nerve terminates in this portion
of the internal ear. Three distinct divisions of the labyrinth have been
made ou