the ear still to be described, namely, the
Eustachian tube, so called from Eustachius, the anatomist, who first
described it. This tube opens by a wide elliptical aperture into the
tympanum behind the membrane; the other end, which gradually grows
wider, opens into the cavity of the mouth. By this canal the inspired
air enters the tympanum to be changed and renewed, it likewise serves
some important purpose in hearing, with the nature of which we are
yet unacquainted. It is certain that we can hear through this
passage, for if a watch be put into the mouth, and the ears stopped,
its ticking may be distinctly heard; and in several instances of
deafness, this tube has been found completely blocked up.

The waves, which have been described as propagated in the air, in all
directions from the sounding body, enter the external cartilaginous
part of the ear, which, as has before been observed, is admirably
fitted for collecting and condensing them. As soon as these pulses
excite tremors in the membrane of the tympanum, its muscles stretch
and brace it, whence it becomes more powerfully affected by these
impulses. It is on this account that we hear sounds more distinctly
when we attend to them, the membrane being then stretched.

A tremulous motion, being excited in this membrane, is communicated
to the malleus annexed to it, which communicates it to the incus, by
which it is propagated through the os orbiculare to the stapes, which
imparts this tremulous motion through the foramen ovale to the fluid
contained in the labyrinth. This tremor is impressed by the waves
excited in this fluid, on every part of the auditory nerve in the
labyrinth. The use of the foramen rotundum, or round hole, before
described, is probably the same as that of the hole in the side of a
drum; it allows the fluid in the labyrinth to be compressed,
otherwise it could not vibrate.

If the organization is sound, and tremors are communicated to the
auditory nerve, they are in some way or other conveyed to the mind,
but in what manner we cannot tell. Nature has hid the machinery by
which she connects material and immaterial things entirely from our
view, and if we try to investigate them, we are soon bewildered in
the regions of hypothesis.

Tremors may however be communicated to the auditory nerve in a
different manner from what I have described. If a watch be put
between the teeth, and the ear stopped, tremors will be communicated
to the teeth, by th