ntral point, (5.) 7, The central portion of
the labyrinth, (vestibule.) 8, The foramen rotundum. 9, The fenestra
ovalis. 11, 12, 13, 14, 15, 16, 17, 18, The semicircular canals. The
cochlea and semicircular canals open into the vestibule.]

954. The EUSTACHIAN TUBE is a canal of communication, extending
obliquely between the pharynx and the anterior circumference of the
tympanum. In structure it is partly fibro-cartilaginous and partly
bony. It is broad and expanded at its pharyngeal extremity, and narrow
and compressed at the tympanum.

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953. Where are the mastoid cells? Explain fig. 146. 954. Describe the
Eustachian tube.

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955. The small openings of the middle ear are for the entrance and
exit of the chorda tympani, (a small nerve that crosses the tympanum,)
and for the exit of the muscles that act upon the membrana tympani and
bones of the ear.

956. The LABYRINTH consists of a membranous and a bony portion. The
bony labyrinth presents a series of cavities which are channelled
through the substance of the petrous bone. It is situated between the
cavity of the tympanum and the _Aud'it-o-ry Nerve_. The labyrinth is
divided into the _Ves'ti-bule_, _Sem-i-cir'cu-lar Canals_, and
_Coch'le-a_.

957. The VESTIBULE is a small, three-cornered cavity, situated
immediately within the inner wall of the tympanum.

958. The SEMICIRCULAR CANALS are three bony passages which communicate
with the vestibule, into which two of them open at both extremities,
and the third at one extremity.

959. The COCHLEA forms the anterior portion of the labyrinth. It
consists of a bony and gradually tapering canal, about one and a half
inches in length, which makes two turns and a half, spirally, around a
central axis, called the _mo-di'o-lus_. The modiolus is large near its
base, where it corresponds with the first turn of the cochlea, and
diminishes in diameter toward its extremity.

960. The interior of the canal of the cochlea is partially divided
into two passages, by means of a bony and membranous plate. At the
extremity of the modiolus, the two passages communicate with each
other. At the other extremity, one opens into the vestibule; the other
into the tympanum, by the foramen rotundum. The internal surface of
the bony labyrinth is lined by a fibro-serous membrane.

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955. What passes through the small openings of the middle ear? 956. Of
what do