h have been directed
inwards by the external ear, strike, and cause it to vibrate like the
membrane of a drum. This membrane is stretched over a cavity in the
bone, called the os petrosum, which cavity is called the tympanum, or
drum of the ear, which is of a rounded figure, divided in its middle
by a promontory, and continued backwards to the cells of the mastoid
bone. Besides this continuation of the tympanum into the mastoid
cells, it has a free communication with the mouth, by means of a tube
I shall soon describe.

Within this cavity of the tympanum are placed four small bones, which
facilitate the hearing: the first is the malleus or hammer, so called
from its shape: the upper part of its round head rests upon the
concavity of the tympanum, from whence the handle is extended down,
along the membrane of the tympanum; this bone has several muscles,
which move it in different directions, and cause it to stretch or
brace the membrana tympani, when we wish to hear with accuracy.

Connected with the malleus is another small bone, called the incus,
or anvil, which is connected with another, called the stapes, or
stirrup, from its shape. These two bones are connected by a small
oval shaped bone, called os orbiculare, placed between them: the
whole forming a little chain of bones.

The stapes, or stirrup, has its end of an oval shape, which fits a
small hole called fenestra ovalis, in that part of the ear called the
labyrinth, or innermost chamber of the ear.

The labyrinth consists of three parts; first, the vestibule, which is
a round cavity in a hard part of the os petrosum; secondly, the
semicircular canals, so called from their shape, which however is not
exactly semicircular; thirdly, the cochlea, which is a beautifully
convoluted canal, like the shell of a snail. This part has a round
cavity called fenestra rotunda, which is covered with a thin elastic
membrane, and looks into the tympanum.

The vestibule, semicircular canals, and cochlea, the whole of which
is called the labyrinth, form one cavity, which is filled with a very
limpid fluid resembling water, and the whole lined with a fine
delicate membrane, upon which the auditory nerve is expanded, like
the retina upon the vitreous humor of the eye. This beautiful
apparatus was only lately discovered by an Italian physician, Scarpa.
The auditory nerve is a portion of the seventh pair, which is called
the portio mollis or soft portion.

There is one part of