Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

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ANATOMY.
Special oblique portion, and an inferior, narrow, horizontal one;
Anatomy, the fossa navicularis; the tragus, an anterior eminence
below; the antitragus, a smaller eminence behind; the
lobule, a pendulous body at the base behind; and, lastly,
the concha, a deep conoidal cavity leading to the ear-hole.
The latter is a canal about 10 or 12 lines long in the
adult. Twisted at first obliquely forward and upward,
it bends slightly backwards and downwards, forming a
convexity of incurvation above, and a concavity below.
Though the extremities are large, the middle is contract¬
ed ; and it cannot be termed cylindrical, for its section is
elliptical or oval. The structure of this tube is fibro-car-
tilaginous externally where it adheres to the bone, lined
by skin passing into mucous membrane, and occupied by
minute follicles (glandulce ceruminoscc), which secrete the
wax (cerumeri) formed in this canal. The nature of this
secretion is imperfectly known. Though, like oil, it stains
paper, it is partly soluble in tepid water, and forms with
it a yellow emulsion. It is secreted at first fluid, and
acquires consistence by exposure to air and admixture
with dust. Alcohol has little influence on it. The in¬
ternal extremity of the auditory canal is bounded by the
vertical membrane of the tympanum.
Tympanal Within this is the tympanal cavity, a space of an irre-
cavity. gular cylindrical shape, directed obliquely, nearly in the
axis of the pyramidal portion, in the base of which it is
contained. This cavity is shut up externally by the ver¬
tical tympanal membrane (membrana tympani), and is
bounded within by the bony partition which separates it
from the labyrinth. The membrana, which is oval-shaped,
or nearly round, and attached to the margin of the mea¬
tus externus, is directed obliquely downwards and inwards,
and is so delicate that it is difficult to determine its struc¬
ture in the human subject. In the elephant, however, and
other large animals, it presents radiating fibres, which are
believed to be muscular (Plate XXXVII. fig. 14) ; and Sir
E. Home represents it as such not only in the elephant
and whale, but in the human subject. The outer part is
evidently a sort of epidermis, continuous with that of the
canal; the inner is a mucous epidermis, continuous with
that of the tympanal cavity; and between these the mus¬
cular fibres are interposed.
Eustachian The tympanal cavity communicates behind with the
tube. mastoid cells, and before and internally by the Eus¬
tachian tube, with the pharynx. This tube is estimated
to be two inches in length, of which one and a half
is in the bone of the pyramid, and about half an inch at
its extremity, with the upper side completed by carti¬
lage. Narrow at the tympanal end, it becomes wide and
capacious towards the pharyngeal, and presents at length
a free open extremity, forming a fissure at the upper and
lateral part of the pharynx. The cartilaginous end is
covered by mucous membrane continuous with the pha¬
ryngeal, and is surrounded by the peristaphylini, the action
of which is believed to separate the walls of the aperture.
Within the tube, and towards the tympanal end, this mem¬
brane parts with its pharyngeal spongy character, and be¬
comes thin and semitransparent where it lines the bone.
The same kind of membrane, partaking of the characters
of periosteum and mucous, is continued over the tympanal
cavity, and into the mastoid cells.
Muscle of Above the Eustachian tube is a thin osseous plate, which
the malleus separates it from a small canal, convex below, concave
and groove, above, and which, commencing in the fissure between the
squamous and pyramidal portions, terminates in the tym¬
panal cavity. In this canal is lodged the internal muscle
of the malleus, one of the tympanal bones.
Tympanal These are four in number, very minute, and denomi-
oones. nated, from their mechanical figures, the hammer (mal¬
leus), the anvil (incus), the lenticular or round bone (os Special
orbiculare), and the stirrup (stapes). Of these the ma//ews is Anatomy
attached to the vertical membrane by its handle, while its
head is articulated with the body of the incus. The latter
presents two limbs or branches, to the larger of which the
stapes is articulated by the interposition of the lenticular
bone; while the base of the former rests on the membrane of
the foramen ovale. These articulations are secured by cap¬
sules, which allow the bones to move freely on each other ;
and for this purpose the stapes is provided with one muscle,
and the malleus with two, an internal already mentioned,
and an external passing from the spinous process of the
sphenoid bone to the slender process of the malleus. On
the motions of these, however, and their part in the pro¬
cess of hearing, we have only conjectural statements.
The internal bony wall of the tympanal cavity presents Foramen
two apertures and a convex intermediate eminence. Of ovale,
the apertures, the first, which is named the oval or vesti-fe™jftra
bular aperture (foramen ovale, fenestra ovalis), is situate °'a 1S'
above, oval transversely, with its great diameter horizontal
antero-posterior. It communicates with the vestibule, but
is closed bv a fine membrane, to which the base of the
stapes is fixed, and for the insertion of which its margin is
grooved. The oval aperture is bounded above by a round Promon-
prominence, corresponding within to the Fallopian aque-tory.
duct, and below by a large convex eminence named the
promontory (promontorium), which indicates the situa¬
tion of the cavity named the vestibule. Before and above
the promontory is the extremity of the thin osseous plate
which separates the Eustachian tube from the canal of the
internal muscle of the malleus; and behind is an oblique
cavity, which is placed between the lower entrance of the
mastoid cells and the pyramid. Below the promontory is
the round or cochlear aperture (foramen rotundum, fenestra
rotunda), trilateral in early life rather than round, and
still preserving in the adult the tendency to this shape ;
smaller than the oval, and directed backwards and out¬
wards. The round aperture is shut by a membrane, the
direction of which is oblique to that of the tympanum, and
one side of which is towards that cavity, while the other
forms part of the cochlea.
At the upper part of the tympanum is a triangular-Mastoid
shaped opening, which leads into a rough short canal, cells. j
terminating in the mastoid cells. These are analogous
to the cells of the ethmoid, sphenoid, and occipital bones.
They are lined by fibro-mucous membrane, and their use
is to afford a posterior sonorous apartment for the vibra¬
tions produced in the tympanal cavity.
Near this triangular opening is a small bony process
named the pyramidal, in which is a canal for the fleshy
part of the stapedius, while the tendon issues from its
orifice. Near the base of the pyramidal process is the hole
by which the nerve of the tympanum (chorda tympani)
passes through the glenoid fissure.
The labyrinth consists of the vestibule, three semicircu-Labyrinth,
lar canals, and the cochlea.
By removing the stapes and stapedial membrane the Vestibule,
oval aperture is opened, and communicates with the vesti¬
bule. This cavity, which is irregular in shape, about the
size of a grain of barley, is bounded without by the tym¬
panum, within by the internal auditory canal, before by
the cochlea, behind by the semicircular canals, and above
and below by the solid bone of the pyramid. It is lined
by a membrane common to the whole labyrinth. Besides
the oval aperture by which it is separated from the tympa¬
num, it has, above, the two anterior openings (ampullulce)
of the superior vertical and horizontal canals ; behind, the
two openings (ampullulce) proper to the posterior verti¬
cal and horizontal canals, and the common opening of the