Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
Special two vertical canals; and before and below, the orifice of the
Anatomy, external scala cochlea.
There is still another aperture, which leads into a canal
Aqueduct discovered by Cotugno, named the aqueduct of the ves-
tibule ami" tibulc- This, which, though distinct in some subjects, is
aperture, almost imperceptible in others, is near the common orifice
of the vertical canals; and from it the aqueduct proceeds
first upwards, where it is narrow, then backwards and
downwards, widening, and terminates in the fissure on
the posterior surface of the pyramidal portion.
Semicircu- The semicircular canals, situate behind the vestibule,
lar canals. are three in number, two vertical and one horizontal. Of
the former, one is superior, inclosing by its curvature the
substance of the pyramid, and forming a convexity in the
adult, very distinct in the foetus ; while the other, which is
posterior but inferior, is placed with its plane correspond¬
ing to that of the posterior surface of the pyramid. The
third is placed horizontally between the other two, form¬
ing a curvature with the convexity towards the base of the
pyramid. (Plate XXXIII. fig. 8, 9, and 10.)
Though denominated semicircular, these canals are
larger than semicircular, and may be compared to hollow
cylinders, incurvated so as to form large circular segments.
Each canal has an enlarged extremity named ampullula ;
and as these two vertical canals have one in common, there
are five ampulluloe. They are lined by the common la¬
byrinthine membrane, and contain a pellucid fluid.
Cochlea. The cochlea, which forms the third part of the labyrinth,
is a conical canal turned spirally within itself, so that its
base is at the lower part of the vestibule, and its apex at
the anterior side of the pyramid, with the orifices for the
auditory nerve inclosed in the centre of its turns, while
the convexity is directed towards the lower margin of the
pyramid. The cochlear canal is divided longitudinally by
a thin sharp-edged plate, half bony half membranous, into
two independent cavities, the superior of which communi¬
cates with the vestibule, while the inferior is bounded by
the membrane of the round aperture. These cavities are
distinguished as the vestibular and tympanal {scala vestibuli
and scala tympani) respectively. At the top they termi¬
nate in a common cavity named the funnel {infundibulum).
Both are lined by a delicate membrane, in which are con¬
tained the ramified filaments of the eighth or auditory
nerve. fPlate XXXIII. fig. 10.)
;Aqueduct In the tympanal scala, near the round hole, is a minute
of the aperture leading to a narrow canal, which gradually en-
cochlea. larges as it ascends, till it terminates by a slit on the pos¬
terior surface of the pyramidal portion, as formerly men¬
tioned. This is the aqueduct of the cochlea, first describ¬
ed, like that of the vestibule, by Cotugno.
Blood-ves- These different cavities are supplied with blood chiefly
sels ot the from minute branches of the auditory, a vessel generally
derived from the basilar trunk or the vertebral arteries.
From the meningeal artery also minute branches enter the
auditory canal, and anastomose with those of the auditory
artery; and the internal carotid sends to the membrane
of the tympanal cavity a branch, the capillary ramifica¬
tions of which anastomose with those derived from the
pharyngeal, transmitted by the walls of the Eustachian
tube.
uditory The eighth or the proper auditory nerve enters the coch-
erve- lea by several minute apertures in the internal meatus,
and is divided into two fasciculi, of which the posterior and
largest is expanded in the form of soft pulpy brush-like
filaments, like a hair-pencil, in a pellucid fluid, in the coch¬
lea ; while the smallest, which is anterior, is distributed
partly to the bottom of the hemispherical cavity of the
vestibule, partly to the beginning of the spiral lamina.
(Fig. 10.)
The tympanal cavity is chiefly for the purpose of con- Special
veying and augmenting the intensity of sonorous vibra- Anatomy,
tions, while the shape of the auricle is supposed to collect
them. But of the mechanism of its operation we know
nothing satisfactory. The essential part of the organ of
hearing is the labyrinth.
SECT. IV.—THE ORGAN OF TASTE.
It is impossible to define the exact limits of the sense
of taste. More or less diffused over the cavity of the
mouth, it is particularly confined to the tongue and palate.
The former, nevertheless, is remarkable for being a mus¬
cular organ, which combines with the faculty of taste
the power of prehension and transmission of the alimen¬
tary articles both during and after mastication, and is
further an essential agent in the faculty of articulation.
It is requisite, therefore, to give a short account of the
mouth, the palate, and the tongue.
a. The mouth is the cavity formed by the lips before, the The
pharynx and isthmus faucium beMvad., the palatine vault mouth-
above, the intramaxillary membrane and muscles below,
and on the sides by the cheeks. Its horizontal direction
may be regarded as one among other proofs of the neces¬
sity of the erect biped attitude.
The mouth is lined by a mucous membrane, soft, spongy,
red, and vascular. It may be traced from the inner or
alveolar surface of the lips to the inner surface of the
cheeks on each side over the gums, where it is continuous
with that of the alveolar folliculi, over the inner surface of
each maxillary ramus, and the attached muscles and glands
below, until it is identified with that of the tongue, and
above over the palatine vault back to the uvula. In these
several points, though its organization is the same, its
mechanical arrangement varies considerably as the parts
are fixed or movable. Thus, on the palatine vault and at
the gums it is tense, and adheres pretty firmly to the fibrous
layers forming the periosteum of these parts. In the
angular space between the lips and gums, however, in
that between the inner surface of the alveolar arch, and
all over the lower part of the mouth, where it is connect¬
ed to the inferior surface of the tongue, it is extensive,
loose, moves easily over an abundant layer of filamentous
tissue, and is generally disposed in irregular folds. Ad¬
hering to the internal spine of the lower jaw, a fold or du-
plicature containing condensedfilamentous tissue is reflect¬
ed, to be attached to the median line of the tongue, and
forms the frenum of that organ. In all points it abounds
with muciparous follicles. It also presents on each side
of the tongue the orifices of the sublingual glands.
The mouth has two outlets, an anterior or facial formed Outlets of
by the lips (labia), and a posterior or pharyngeal formed
by the velum palatinum and its appendages.
The lips are musculo-membranous folds, attached all The lips,
round to the superior and inferior jaw-bones, above and
below the alveolar arches, and parted by a transverse
opening or fissure into upper and lower, with right and
left commissures or angles (canthi). In the Negro race
they are particularly bulky and flaccid, with their free
margins (prolabia) much everted; but in the Asio-Euro-
pean they are thinner, and more constricted.
Each lip consists externally of skin continuous with
that of the face, internally of mucous membrane continu¬
ous with that of the mouth, with interposed filamentous
tissue and muscles, well supplied by blood-vessels and
nerves.
The point of union between the skin and oral mucous
membrane is marked by a rounded edge, covered by a thin,
vermilion-red, soft and delicate pellicle, extending between