THE SENSE OF HEARING.
943. The sense of hearing is next in importance to that of vision.
Through this sense we are enabled to perceive sounds, that not only
subserve to our comfort and pleasure, but are instrumental in
promoting our intellectual enjoyments. The organ of hearing, or the
ear, is one of the most complicated in the human body.
ANATOMY OF THE ORGANS OF HEARING.
944. The EAR is composed of three parts: 1st. The _External Ear._ 2d.
The _Tym'pan-um_, or middle ear. 3d. The _La'by-rinth_, or internal
ear.
945. The EXTERNAL EAR is composed of two parts: The _Pin'na_,
(pavilion of the ear,) and the _Me-a'tus Aud-it-o'ri-us Ex-ter'nus_,
(auditory canal.)
946. The PINNA is a cartilaginous plate which surrounds the entrance
of the auditory canal. It presents many ridges and furrows, arising
from the folds of the cartilage that form it.
_Observation._ The pinna, in many animals, is movable; in those that
pursue their prey, it is generally directed forward; in timid animals,
as the hare and rabbit, it is directed backward. In man, this part is
but slightly under the control of the will.
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943. What is said of the importance of hearing? Is the ear complicated
in its structure? 944-962. _Give the anatomy of the organs of
hearing._ 944. Of how many parts is the ear composed? Name them. 945.
Give the parts of the external ear. 946. Describe the pinna. What is
said in reference to the pinna of many animals?
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947. The MEATUS AUDITORIUS is a canal partly cartilaginous, and partly
bony, about an inch in length, which extends inward from the pinna to
the _Mem'bra-na Tym'pan-i_, (drum of the ear.) It is narrower in the
middle than at the extremities. It is lined by an extremely thin pouch
of cuticle, which, when withdrawn, after maceration, preserves the
form of the canal. Some stiff, short hairs are also found in the
interior of the channel, which stretch across the tube, and prevent
the ingress of insects. Beneath the cuticle are a number of small
follicles, which secrete the wax of the ear.
[Illustration: Fig. 144. A representation of the four bones of the ear.
The smallest is highly magnified. This bone is early matured, and in the
adult it becomes united with the incus. These bones are retained in their
places and moved by three ligaments and four muscles.]
948. The MEMBRANA TYMPANI is a thin, semi-transparent membrane, of an
oval shape. It is about three eighths of an inch in diameter, and is
inserted into a groove around the circumference of the meatus, near
its termination. This membrane is placed obliquely across the area of
that tube. It is concave toward the meatus, and convex toward the
tympanum.
949. The TYMPANUM consists of an irregular bony cavity, situated
within the temporal bone. It is bounded externally by the membrana
tympani; internally by its inner wall; and in its circumference by the
petrous portion of the temporal bone and mastoid cells. The tympanum
contains four small bones, called the _os-sic'u-la au-di'tus_. These
are named separately, the _mal'le-us_, _in'cus_, _sta'pes_, and
_or-bic'u-lar_.
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947. What is the meatus auditorius? What is found in this canal? What
is their use? Where is the wax of the ear secreted? 948. Describe the
membrana tympani. 949. Where is the tympanum situated?
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950. There are ten openings in the middle ear; five large and five
small. The larger openings are, the _Me-a'tus Aud-it-o'ri-us
Ex-ter'nus_, _Fe-nes'tra O-va'lis_, (oval window,) _Fe-nes'tra
Ro-tun'da_, (round window,) _Mas'toid Cells_, and _Eu-sta'chi-an
Tube_.
[Illustration: Fig. 145. A representation of the pinna, meatus, membrana
tympani, bones of the ear, and semicircular canals. _a_, The pinna. _c_,
The meatus auditorius externus. _g_, The membrana tympani. _k_, The
tympanum. _e_, The bones of the ear. _b_, The semicircular canals. _f_,
The cochlea. _h_, The vestibule. _i_, The Eustachian tube. _d_, The
auditory nerve.]
951. The FENESTRA OVALIS is the opening of communication between the
tympanum and the vestibule. It is closed by the foot of the stapes, or
bone of the ear, and by the lining membrane of both cavities.
952. The FENESTRA ROTUNDA serves to establish a communication between
the tympanum and the cochlea. it is closed by a proper membrane, as
well as by the lining of both cavities.
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What does this cavity contain? 950. How many openings in the tympanum?
Explain fig. 145. 951. Describe the fenestra ovalis. 952. The fenestra
rotunda.
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953. The MASTOID CELLS are very numerous, and occupy the whole of the
interior of the mastoid process of the temporal bone, and part of the
petrous bone. They communicate, by a large, irregular opening, with
the upper and posterior circumference of the tympanum.
[Illustration: Fig. 146. A view of the labyrinth laid open. This figure
is highly magnified. 1, 1, The cochlea. 2, 3, Two channels, that wind two
and a half turns around a central point, (5.) 7, The central portion of
the labyrinth, (vestibule.) 8, The foramen rotundum. 9, The fenestra
ovalis. 11, 12, 13, 14, 15, 16, 17, 18, The semicircular canals. The
cochlea and semicircular canals open into the vestibule.]
954. The EUSTACHIAN TUBE is a canal of communication, extending
obliquely between the pharynx and the anterior circumference of the
tympanum. In structure it is partly fibro-cartilaginous and partly
bony. It is broad and expanded at its pharyngeal extremity, and narrow
and compressed at the tympanum.
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953. Where are the mastoid cells? Explain fig. 146. 954. Describe the
Eustachian tube.
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955. The small openings of the middle ear are for the entrance and
exit of the chorda tympani, (a small nerve that crosses the tympanum,)
and for the exit of the muscles that act upon the membrana tympani and
bones of the ear.
956. The LABYRINTH consists of a membranous and a bony portion. The
bony labyrinth presents a series of cavities which are channelled
through the substance of the petrous bone. It is situated between the
cavity of the tympanum and the _Aud'it-o-ry Nerve_. The labyrinth is
divided into the _Ves'ti-bule_, _Sem-i-cir'cu-lar Canals_, and
_Coch'le-a_.
957. The VESTIBULE is a small, three-cornered cavity, situated
immediately within the inner wall of the tympanum.
958. The SEMICIRCULAR CANALS are three bony passages which communicate
with the vestibule, into which two of them open at both extremities,
and the third at one extremity.
959. The COCHLEA forms the anterior portion of the labyrinth. It
consists of a bony and gradually tapering canal, about one and a half
inches in length, which makes two turns and a half, spirally, around a
central axis, called the _mo-di'o-lus_. The modiolus is large near its
base, where it corresponds with the first turn of the cochlea, and
diminishes in diameter toward its extremity.
960. The interior of the canal of the cochlea is partially divided
into two passages, by means of a bony and membranous plate. At the
extremity of the modiolus, the two passages communicate with each
other. At the other extremity, one opens into the vestibule; the other
into the tympanum, by the foramen rotundum. The internal surface of
the bony labyrinth is lined by a fibro-serous membrane.
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955. What passes through the small openings of the middle ear? 956. Of
what does the labyrinth consist? Give the parts of the internal ear.
957. Describe the vestibule. 958. What is said of the semicircular
canals? 959. Why is the cochlea so called? Of what does it consist?
960. How is the interior of the canal of the cochlea divided? Where do
they communicate with each other?
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961. The membranous labyrinth is smaller in size, but a perfect
counterpart, with respect to form, of the bony vestibule, cochlea, and
semicircular canals. Within this labyrinth are two small, elongated
sacs, which are filled with a fluid.
[Illustration: Fig. 147. A view of the auditory nerve. 1, The spinal
cord. 2, The medulla oblongata. 3, The lower part of the brain. 4, The
auditory nerve. 5, A branch to the semicircular canals. 6, A branch to
the cochlea.]
962. The AUDITORY NERVE enters the temporal bone upon its internal
surface, and divides into two branches, at the bottom of the cavity of
the internal ear. These branches enter the structure of the elongated
sacs and membranous labyrinth, radiating in all directions, and
finally, they terminate upon the inner surface of the membrane, in
minute papillae, resembling those of the retina.
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By what is the internal labyrinth lined? 961. Describe the membranous
labyrinth. What does fig. 147 represent? 962. Where does the auditory
nerve enter and divide? Where do the branches of the auditory nerve
enter and terminate?
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