gristly portion of the auditory apparatus is about one inch in length,
lined by a continuation of the integument of the ear, and has numerous
hairs on its surface, to prevent the intrusion of foreign substances.
Between the external MEATUS and the cavity of the middle ear is the
_membrana tympani_, which is stretched across the opening like the head
of a drum. The _tympanum_, or ear-drum, communicates with the pharynx by
the _eustachian tube_, which is a narrow passage lined with delicate,
ciliated epithelium. On the posterior portion it is connected with the
_mastoid cells_. Three small bones are stretched across the cavity of
the tympanum, and called, from their form, the _malleus, incus_ and
_stapes_, or the hammer, anvil, and stirrup. Agassiz mentions a fourth,
which he terms the _os orbiculare_. Each wave of sound falling upon the
membrana tympani, throws its molecules into vibrations which are
communicated to the chain of bones, which, in turn, transmits them to
the membrane of the _foramen ovale_. The three muscles which regulate
the tension of these membranes are termed the _tensor tympani, laxator
tympani_, and _stapedium tympani._

The _Labyrinth_, or _Internal_ ear, is a complicated cavity, consisting
of three portions termed the _vestibule, cochlea_, and _semi-circular
canals_. The vestibule is the central portion and communicates with the
other divisions. The labyrinth is filled with a transparent fluid,
termed _perilymph_, in which are suspended, in the vestibules and
canals, small membranous sacs, containing a fluid substance, termed
_endolymph_ (sometimes called _vitrine auditive_ from its resemblance to
the vitreous humor of the eye). The filaments of the auditory nerve
penetrate the membranous tissues of these sacs, and also of those
suspended at the commencement of the semi-circular canals. These little
sacs are supposed to be the seat of hearing, and to determine, in some
mysterious way, the quality, intensity and pitch of sounds.