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 ce