ÏÏEAEING
sac, the membranous labyrinth, fitting loosely
the bony chamber, and separated from it by a
lymph space, whose fluid, the perilymph, does
not communicate with the similar fluid, or
endolymph, within the membranous labyrinth.
The parts of the membranous labyrinth are
the utriculus, into which the semicircular
canals open, each with an expansion or ampulla
at one end, the sacculus, the recessus labyrinthi,
and the cochlea. Their relations are shown
by the accompanying Figs. 4 and 5. Each of
these, except the recessus, contains a patch of
sensory epithelium. The morphology and
functions of these parts can be better under¬
stood after a rapid survey of their phylogenetic
and embryological development.
In the fishes there is associated with the
ear an elaborate system of cutaneous and sub¬
cutaneous sense organs, the lateral line organs,
whose canals and patches of sensory epithe¬
lium resemble very closely the semicircular
canals and their cristae in the internal ear.
The ear of these animals, moreover, lacks the
cochlea and organ of Corti, the corresponding
part of the labyrinth (lagena) being provided
with a simple sensory spot like the cristae of
the semicircular canals. The ear and lateral
line organs not only resemble each other in
structure, but they arise embryologically from
the same area of thickened ectoderm, and
they have been shown experimentally to have
similar functions, viz. the regulation of the
bodily equilibrium. In these animals the
sense of hearing is very feebly developed,
some authors even going so far as to deny its
presence altogether. It is obvious, however,
that a structure adapted to perceive simple
impulses in a fluid medium, such as must be
mediated by an organ of equilibration, will
also be able to respond to vibratory impulses
of low frequency in the same medium. As a
matter of fact, we know that fishes, though
they may be deaf to sound-waves of the higher
frequencies, are nevertheless very sensitive to
mechanical shocks, such as passing footfalls.
Now, the terrestrial animals require much less
elaborate organs of bodily equilibrium than do
the aquatic animals ; but, on the other hand,
they find themselves in an environment of
aerial vibrations which are of great impor¬
tance to their vital economy. Accordingly
the lateral line organs disappear in verte¬
brates higher than the Amphibia, the semi¬
circular canals alone being sufficient for the
static sense, while a portion of the sacculus,
the lagena, progressively increases in com¬
plexity until in the mammals it becomes the
cochlea. Parallel witb these changes the
sound-conducting apparatus of the middle ear
is gradually evolved. It thus appears that
there are two distinct sense organs in the
internal ear, organs which have had a common
origin, and which, even in man, may be only
incompletely differentiated from each other ;
viz. the vestibule and semicircular canals for
the sense of equilibrium, and the cochlea, the
organ of hearing.
In its embryological development, the human
ear first appears as a thickened bit of ecto¬
derm at the side of the medulla oblongata,
which soon becomes depressed to form the
‘auditory saucer.’ At a later period this
saucer is completely invaginated to form the
‘ auditory vesicle,’ retaining, however, for a
time its communication with the outer surface
of the body. This condition is permanent in
the sharks, the endolymph of the labyrinth
communicating with the outer sea-water by
the endolymphatic duct, just as the lateral
line canals freely communicate with the sur¬
face by pores. In higher animals this con¬
nection is early lost, though the recessus
labyrinthi is regarded as a vestige of that
connection. The lining membrane of the
auditory vesicle develops a patch of sensory
epithelium, and as the vesicle becomes con¬
stricted into the several chambers comprising
the labyrinth, an extension of this sensory patch
grows into each chamber. These sensory
areas then become separated by indifferent
epithelium, and thus arise the three cristae in
the ampullae, the macula utriculi, the macula
sacculi, and the organ of Corti. The structure
of all of these sensory organs except the organ
of Corti is similar and very simple, the specific
sensory cells being shorter than the indiffer¬
ent cells, and provided with hairs which pro¬
ject into the endolymph. The base of these
cells is embraced by the terminal arborizations
of the corresponding nerve-fibres (see Fig. 6).
This structure is essentially similar to that of
the sensory organs in the lateral line canals of
fishes.
The bony cochlea is formed somewhat like
the interior of a snail-shell, with two and one-
half turns of the spiral, and with a bony shelf
or ledge, the lamina spiralis, extending out¬
ward from the axis, or modiolus. Within the
spirals of the bony cochlea are three mem¬
branous canals : (1) The scala vestibuli com¬
municates at its base with the perilymphatic
space around the vestibule, whose fluid is
caused to vibrate by the foot of the stapes,
which plays in the fenestra ovalis between
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