The =middle ear= (Auris media) or =tympanic cavity= (Cavum tympani) is a
space in the tympanic and petrous parts of the temporal bone situated
between the membrana tympani and the internal ear. It is a pneumatic
cavity, which is lined by mucous membrane, and communicates with the
pharynx and the guttural pouch by the Eustachian tube. It contains a
chain of auditory ossicles by which the vibrations of the membrana
tympani are transmitted to the internal ear.
The cavity consists of: (1) A main part or atrium, which lies
immediately to the inner side of the membrana tympani; (2) the recessus
epitympanicus, situated above the level of the membrane and containing
the upper part of the malleus and the greater part of the incus; (3) a
relatively large ventral recess in the bulla ossea.
The =external wall= (Paries membranacea) is formed largely by the thin
=membrana tympani=, which closes the inner end of the external auditory
canal, and thus forms the septum between the external and middle parts
of the ear. The membrane is an oval disc, which slopes obliquely
downward and inward at an angle of about 30 degrees with the lower wall
of the external auditory canal. The circumference is attached in a
groove (Sulcus tympanicus) in the thin ring of bone (Annulus tympanicus)
which almost completely surrounds it. The handle of the malleus (the
outermost of the auditory ossicles) is attached to the inner surface of
the membrane and draws the central part inward, producing a slight
concavity of the outer surface. The periphery is thickened, forming the
annulus fibrosus. The membrana tympani consists of three layers. The
external =cutaneous layer= (Stratum cutaneum) is a prolongation of the
lining of the external auditory meatus. The middle =fibrous layer= or
=membrana propria= includes two chief sets of fibers; the outer stratum
(Stratum radiatum) consists of fibers which radiate from the handle of
the malleus, while the inner stratum (Stratum circulare) is composed of
circular fibers which are best developed peripherally. There are also
branched or dendritic fibers in part of the membrane. The internal
=mucous layer= (Stratum mucosum) is a part of the general mucous
membrane which lines the tympanic cavity.
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