_1_, External auditory meatus; _2_, annulus tympanicus; _3_, laminæ;
radiating from _2_; _4_, membrana tympani; _5_, malleus; _6_, incus;
_7_, stapes; _8_, fenestra cochleæ (s. rotundum); _9_, facial canal;
_11_, _11″_, semicircular canals; _11′_, vestibule; _12_, cochlea.
(After Ellenberger, in Leisering’s Atlas)
]
[Illustration:
FIG. 566.—RIGHT AUDITORY OSSICLES AND MEMBRANA TYMPANI, ENLARGED AND
VIEWED FROM INNER SIDE AND BELOW.
_1_, Malleus; _2_, incus; _2′_, _2″_, short and long processes of _2_;
_3_, stapes; _4_, tensor tympani; _5_, ligament attaching long
process of malleus; _6_, stapedius muscle; _7_, fenestra cochleæ (s.
rotundum). (After Ellenberger, in Leisering’s Atlas.)
]
The =internal wall= of the tympanic cavity (Paries labyrinthica)
separates it from the internal ear; it presents a number of special
features. The =promontory= (Promontorium) is a distinct eminence near
the center which corresponds to the first coil of the cochlea, and is
marked by a faint groove for the superficial petrosal nerve. Above this
is the =fenestra vestibuli= (s. ovalis), a reniform opening which is
closed by the foot-plate of the stapes and its annular ligament. The
=fenestra cochleæ= (s. rotundum) is situated below and behind the
preceding; it is an irregularly oval opening and is closed by a thin
membrane (Membrana tympani secundaria), which separates the tympanic
cavity from the scala tympani of the cochlea.
The =anterior wall= (Paries tubaria) is narrow, and is pierced by the
slit-like tympanic opening of the Eustachian tube. Above this and
incompletely separated from it by a thin plate of bone is the semicanal
for the tensor tympani muscle.
The =superior wall= or =roof= (Paries tegmentalis) is crossed in its
inner part by the facial nerve; here the facial canal is more or less
deficient ventrally, and the nerve is covered by the mucous membrane of
the tympanum.
The =posterior wall= (Paries mastoidea) presents nothing of importance;
a tympanic antrum and mastoid cells, such as are found behind the
tympanic cavity proper in man and many animals, are not present in the
horse.
The =inferior wall= or =floor= (Paries tympanica) is concave and thin.
It is crossed by delicate curved ridges, which radiate from the greater
part of the annulus tympanicus.
The =auditory ossicles= (Ossicula auditus) form a chain which extends
from the outer to the inner wall of the cavity. They are named, from
without inward, the =malleus=, the =incus=, the =os lenticulare=, and
the =stapes=. The first is attached to the inner surface of the tympanic
membrane and the last is fixed in the fenestra vestibuli.
[Illustration:
FIG. 567.—AUDITORY OSSICLES AND MEMBRANI TYMPANI, INNER VIEW,
ENLARGED.
_1_, Annulus tympanicus; _2_, membrana tympani; _3_, malleus; _4_,
incus; _5_, stapes. (After Ellenberger, in Leisering’s Atlas.)
]
[Illustration:
FIG. 568.—RIGHT AUDITORY OSSICLES, ENLARGED AS INDICATED BY LINES
GIVING THEIR ACTUAL LENGTH.
_a_, Malleus; _1_, head; _2_, neck; _3_, handle; _4_, long process;
_5_, muscular process. b, Incus; _1_, body; _2_, short branch; _3_,
long branch; _c_, os lenticulare. c, Stapes; _1_, head; _2_, crus;
_3_, base; _4_, attachment of stapedius. (After Ellenberger, in
Leisering’s Atlas.)
]
The =malleus= or hammer, the largest of the ossicles, consists of a
head, neck, handle, and two processes. The =head= (Capitulum mallei) is
situated in the epitympanic recess. It is smooth and convex above and in
front, and presents on its postero-internal aspect a concave facet for
articulation with the body of the incus. The =neck= (Collum mallei) is
the constricted part below the head; its inner surface is crossed by the
chorda tympani. The =handle= (Manubrium mallei) is directed downward,
inward, and a little forward from the neck, and is attached along its
entire length to the membrana tympani. On its inner surface, near the
upper end, there is a slight projection to which the tendon of the
tensor tympani muscle is attached. The =long process= (Processus longus)
is a pointed spicule which projects forward from the neck toward the
petro-tympanic fissure. The =short process= (Processus brevis) is a
slight projection of the outer side of the neck, and is attached to the
upper part of the membrana tympani.
The =incus= or anvil is situated chiefly in the epitympanic recess. It
may be said to resemble a bicuspid tooth with two divergent roots, and
consists of a body and two processes. The =body= (Corpus incudis)
articulates with the head of the hammer. The =long process= (Crus
longum) projects downward from the body and then curves inward; its
extremity has attached to it a small nodule of bone, the os
=lenticulare=, which articulates with the head of the stapes. The =short
process= (Crus breve) projects chiefly backward, and is attached to the
wall of the recess by a small ligament.
The =stapes= or stirrup consists of a head, two crura, and a base. The
=head= (Capitulum stapedis) is directed outward and articulates with the
os lenticulare. The =crura=, anterior and posterior (Crus anterius,
posterius), are directed inward from the head, and join the ends of the
base. The =base= or foot-plate (Basis stapedis) occupies the fenestra
vestibuli, to which it is attached. The space between the crura and the
base is closed by a membrane.
The =articulations= and =ligaments= of the auditory ossicles comprise:
(1) A diarthrodial joint between the head of the malleus and the body of
the incus (Articulatio incudomalleolaris), inclosed by a capsule. (2) An
enarthrosis between the os lenticulare and the head of the stapes
(Articulatio incudostapedia), also surrounded by a capsule. (3) The base
of the stapes is attached to the margin of the fenestra vestibuli by a
ring of elastic fibers (Lig. annulare baseos stapedis). (4) Small
ligaments attach the head of the malleus and the short crus of the incus
to the roof of the epitympanic recess. (5) The axial ligament (of
Helmholtz) attaches the neck of the malleus to a small projection (Spina
tympanica anterior) above and in front of the annulus tympanicus.
The =muscles= of the auditory ossicles are two in number, viz., the
tensor tympani and the stapedius. The =tensor tympani= arises from the
upper wall of the osseous Eustachian tube, and ends in a delicate tendon
which bends outward and is inserted into the handle of the malleus near
its upper end. When it contracts, it draws the handle of the malleus
inward and tenses the membrana tympani; it probably also rotates the
malleus around its long axis. It is innervated by the motor part of the
trigeminus through the otic ganglion. The =stapedius= arises from a
small prominence (Eminentia pyramidalis) of the posterior wall of the
tympanum, runs forward on the facial nerve, and is inserted into the
neck of the stapes. Its action is to draw the head of the stapes
backward and rotate the anterior end of the base outward, thus tensing
the annular ligament. It is innervated by the facial nerve.
The =tympanic mucous membrane= (Tunica mucosa tympanica) is continuous
with that of the pharynx and the guttural pouch through the Eustachian
tube. It is thin, closely united with the underlying periosteum, and is
reflected over the ossicles, ligaments, and muscles, the chorda tympani,
and the facial nerve in the open part of the facial canal. It contains
minute lymph nodules and small mucous glands (Glandulæ tympanicæ). The
epithelium is in general columnar ciliated, but over the membrana
tympani, ossicles, and promontory it is flattened.
The =artery= of the tympanum is the stylo-mastoid, a small vessel which
arises from the posterior auricular branch of the internal maxillary
artery. It enters the tympanum through the stylo-mastoid foramen, and
forms a circle around the tympanic membrane. The =nerves= of the mucous
membrane come from the tympanic plexus.