The ear consists of three principal divisions, viz. the external,
intermediate, and internal ear. The different parts of the first division,
or external ear, are described by anatomists under the name of the helix,
antihelix, tragus, antitragus, the lobe, cavitas innominata, the scapha,
and the concha. In the middle of the external ear is the meatus, or
passage, which varies in length in different individuals. The external
or outward ear is designed by nature to stand prominent, and to bear
its proportion in the symmetry of the head, but in Europe it is greatly
flattened by the pressure of the dress; it consists chiefly of elastic
cartilage, formed with different hollows, or sinuosities, all leading into
each other, and finally terminating in the concha, or immediate opening
into the tube of the ear. This form is admirably adapted for the reception
of sound, for collecting and retaining it, so that it may not pass off, or
be sent too rapidly to the seat of the impression. There have been a few
instances of men who had the power of moving the external ear in a similar
manner to that of animals; but these instances are very rare, and rather
deviations from the general structure; nor did it appear in these
instances that such individuals heard more acutely: a proof that such a
structure would be of no advantage to the human subject. With respect
to the external ear in man, whether it is completely removed either by
accident or design, deafness ensues, although its partial removal is
not attended with this inconvenience: the external ear, therefore, or
something in its form to collect sound, is a necessary part of the organ.
The next division is the intermediate ear; it consists of the tympanum,
mastoid cells, and Eustachian tube. The tympanum contains four small
delicate bones, viz. the malleus, the incus, the stapes, and the os
orbiculare, joined to the incus. The intermediate ear displays an
irregular cavity, having a membrane, called the membrana tympani,
stretched across its extremity; and this cavity has a communication with
the external air, through the Eustachian tube, which leads into the fauces,
or throat. The membrane of the tympanum is intended to carry the
vibrations of the atmosphere, collected by the outward ear, to the chain
of bones which form the peculiar mechanism of the tympanum. Besides the
effect of the hard and bony parts of the ear in increasing the power of
sound, the tension of the different membranes is also a requisite: thus
various muscles are so situated as to put the membrane on the stretch,
that the sound, striking upon it, may, from its tension, similar to that
of the parchment of a drum-head, have full influence upon the sense. In
respect to its tension, the membrane of the tympanum may be also compared,
not unaptly, to the string of a violin, or musical instrument, even more
properly than to a drum; as the state of tension and relaxation in such
chords produces a variety of sound in the instrument, so, in the same
manner, circumstances, which affect the tension and relaxation of the
tympanum, vary most perceptibly its powers of action, and the customary
agency of the organ. 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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