ls_. 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.

The determination of the _direction_ of sound is a problem of acoustics.
Some have contended that the arrangement of the semi-circular canals is
in some way connected with this sensation. But this supposition,
together with the theory of the transmission of sound through the
various portions of the cranial bones, has been exploded.

From the foregoing description, it will be seen that the labyrinth and
tympanum are the most essential parts of the organs of hearing. In
delicacy and refinement this sense ranks next to sight. The emotions of
beauty and sublimity, excited by the warbling of birds and the roll of
thunder, are scarcely distinguishable from the intense emotions arising
from sight. It is a remarkable fact, that the refinement or cultivation
of these senses is always found associated. Those nations which furnish
the best artists, or have the highest appreciation of painting and
sculpture, produce the most skillful musicians, those who reduce music
to a science.

SMELL.

[Illustration: Fig. 65.
1. Frontal sinus. 2. Nasal b