ertical canals, and one is horizontal, the horizontal canal
(h.c.). There are dilatations, called ampullae, at the anterior base of
the anterior, and at the posterior base of the posterior and horizontal
canals. Indirectly connected with the main sac is a spirally-twisted
portion, resembling a snail shell in form, the cochlea. This last part
is distinctive of the mammalia, but the rest of the internal ear is
represented in all vertebrata, with one or two exceptions. The whole of
the labyrinth is membranous, and contains a fluid, the endolymph;
between the membranous wall of the labyrinth and the enclosing bone
is a space containing the perilymph. Strange as it may appear at first,
the entire lining of the internal ear is, at an early stage, continuous
with the general epidermis of the animal. It grows in just as a gland
might grow in, and is finally cut off from the exterior; but a
considerable relic of this former communication remains as a thin,
vertical blind tube (not shown in the figure), the ductus
endolymphaticus.

Section 116. The eighth nerve runs from the brain case (Cr.), into the
periotic bone, and is distributed to the several portions of this
labyrinth. In an ordinary fish this internal ear is the sole auditory organ
we should find; the sound-waves would travel through the water to the
elastic cranium and so reach and affect the nerves. But in all
air-frequenting animals this original plan of an ear has to be added to,
to fit it to the much fainter sound vibrations of the compressible and
far less elastic air. A "receiving apparatus" is needed, and is supplied
by the ear-drum, middle ear, or tympanic cavity (T.). In the mammal
there is also a collecting ear trumpet (the ear commonly so-called),
the external ear, and external auditory meatus (e.a.m.). A tightly
stretched membrane, the tympanic membrane, separates this from
the drum. A chain of small bones, the malleus (m.), the incus (i.), the
os orbiculare (o.or.), a very small bone, and a stirrup-shaped stapes,
swing across the tympanum, from the tympanic membrane to the
internal ear. At two points the bony investment of this last is
incomplete-- at the fenestra rotunda (f.r.), and at the fenestra ovalis,
(f.o.), into which latter the end of the stapes fits, and so
communicates the sound vibrations of the tympanic membrane to the
endolymph. A passage, the Eustachian tube, communicates between
the tympanic cavity and the pharynx (Ph.), and serves to equa