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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