Its four bones act mechanically, in consequence of
the power of the local muscles: they strike like the key of an instrument,
and produce a percussion on the nerves of the tympanum. Not only may the
membrane of the tympanum be partially destroyed, and hearing be preserved,
but the small bones of the tympanum have been in certain cases lost, or
have come away, from ulceration, and through a constitutional or other
cause; but in such cases it appears that the stapes was, in most instances,
left, and thus the openings of the fenestra ovata and fenestra rotunda
were preserved, which prevented the escape of sound from the labyrinth and
internal parts. With respect to the Eustachian tube, its aperture into
the throat seems indispensable to hearing; and whenever closed, from
malconfirmation or disease, deafness is the certain consequence.

The third division of the organ is the internal ear, which is called the
labyrinth; it is divided into the vestibule, three semicircular canals,
and the cochlea: the whole are incased within the petrous portion of the
temporal bone. The internal ear may be considered as the actual seat of
the organ; it consists of a nervous expansion of high sensibility, the
sentient extremities of which spread in every direction, and in the most
minute manner; inosculating with each other, and forming plexus, by which
the auricular sense is increased. Here, also, sound is collected and
retained by the mastoid cells and cochlea. To this apparatus is added the
presence of a fluid, contained in sacs and membranes; as this fluid is in
large quantities in some animals, there is no doubt it is intended as an
additional means for enforcing the impression: the known influence of
water, as a powerful medium or conductor of sound, strengthens this idea.
The internal ear of man, therefore, has all the known varieties of
apparatus, which are only partially present in other classes of the
creation; and its perfection is best judged of, by considering the variety
or form of the internal ear of other animals. The internal ear of some
animals consists of little more than a sac of fluid, on which is expanded
a small nervous pulp; according to the situation of this, whether the
creature lives in water, or is partially exposed to the air, it has an
external opening with the ear, or otherwise.--_Lecture delivered at the
Royal Institution, May 30, 1828--by J.H. Curtis, Esq_.

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TH