hearing, and of certain superadded parts. The essential part is called
the internal ear, and is represented in all the true vertebrata (i.e.,
excluding the lancelet and its allies). In the lower forms it is a hollow
membranous structure, embedded in a mass of cartilage, the otic
capsule; in the mammal the latter is entirely ossified, to form the
periotic bone. The internal ear consists of a central sac, from which
three semicircular canals spring. The planes of the three canals are
mutually at right angles; two are vertical, the anterior and posterior
(p.v.c.) vertical 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 equalize the
pressure on either side of the drum-head. A comparative study of the
ears of the vertebrata brings to light the fact that, as we descend in
the animal scale, the four ear ossicles are replaced by large bones
and cartilages connected with the jaw, and the drum and Eustachian
tube by a gill slit. We have, in fact, in the ear, as the student will
perceive in the sequel, an essentially aquatic auditory organ, added to
and patched up to fit the new needs of a life out of water.