hese
channels (Fig. 153).

*The Scala Vestibula and the Scala Tympani* appear in cross section as the
larger of the canals. The former, so named from its connection with the
vestibule, occupies the upper position in all parts of the coil. The
latter lies below at all places, and is separated from the channels above
partly by a margin of bone and partly by a membrane. It receives its name
from its termination at the tympanum, or middle ear, from which it is
separated only by a thin membrane.(122) Both the scala vestibula and the
scala tympani belong to the outer portion of the internal ear and are, for
this reason, filled with the perilymph. At their upper ends they
communicate with each other by a small opening, making by this means one
continuous canal through the cochlea. This canal passes from the vestibule
to the tympanum and, in so doing, goes entirely around

*The Scala Media.*--This division of the cochlea lies parallel to and
between the other two divisions. It is above the scala tympani and below
the scala vestibula, and is separated from each by a membrane. The scala
media belongs to the membranous portion of the internal ear and is,
therefore, filled with the endolymph. It receives the terminations of
fibers from the auditory nerve and may be regarded as the true sense organ
of hearing. The nerve fibers terminate upon the membrane known as the
_basilar membrane_, which separates it from the scala tympani. This
membrane extends the length of the cochlear canals, and is stretched
between a projecting shelf of bone on one side and the outer wall of the
cochlea on the other. It is covered with a layer of epithelial cells, some
of which have small, hair-like projections and are known as the _hair
cells_. Above the membrane, and resting partly upon it, are two rows of
rod-like bodies, called the _rods of Corti_. These, by leaning toward each
other, form a kind of tunnel beneath. They are exceedingly numerous,
numbering more than 6000, and form a continuous series along the margin of
the membrane.

[Fig. 154]

Fig. 154--*Diagram* illustrating passage of sound waves through the ear.

*How We Hear.*--The sound waves which originate in vibrating bodies are
transmitted by the air to the external ear. Passing through the auditory
canal, the waves strike against the membrana tympani, setting it into
vibration. By the bridge of bones and the air within the middle ear the
vibrations