PISCES. 
turgidly filled with a transparent fluid; the 
membranous vestibule, however, does not ad- 
here to the walls of the capsule except at the 
orifices leading into the cranium. The mem- 
branous vestibule has its cavity divided into 
several compartments by folds projecting into 
its interior, and receives the auditory nerve, 
which being changed into a pulpy mass spreads 
Out over its walls. 
In the Petromyzonidz therefore three 
important parts of the auditory appa- 
ratus, which are met with in the ear 
of all other Fishes, are wanting, viz. 
the sac of the otolithe, the otolithe 
itself, and the semicircular canals, ex- 
cept indeed rudiments of the latter may 
be represented by two curved folds of 
the membrane of the vestibule, which 
are joined: superiorly to a similar fold, 
an arrangement which is met with both 
in the river and sea-lamprey. The 
auditory nerve is derived immediately 
from the brain. 
From the above description it would 
appear that in the Lampreys there are two modes 
whereby sonorous vibrations may be commu- 
nicated to the vestibule, one through the car- 
tilaginous capsule of the ear, the other through 
the cranium, which communicating tremors 
impressed upon it from without to the fluid 
which is contained in its cavity, the vibration 
reaches the tense membrane that closes the 
large fenestra leading to the vestibule, and thus 
affects the membranous vestibular sac itself. 
Inasecond group Weber includes those forms 
of the ear which have no cartilaginous or osse- 
ous vestibule separate from the cranial cavity. 
This kind of ear exists in by far the greater 
number of Fishes, being met with in all the 
truly osseous and branchiostegous races as 
well as in some Chondropterygians; in none 
of which is the membranous labyrinth en- 
closed in a bony or cartilaginous envelope, the 
internal ear being contained in the cavity of 
the skull itself near the posterior part of the 
cerebrum, with which, in fact, it is for the most 
part in apposition; for in these Fishes the 
cranium being very large and having only a 
small part of its cavity occupied by the brain 
itself, performs the office of an osseous laby- 
rinth, not only by furnishing a receptacle to 
the internal ear in which every part necessary 
to the performance of its functions may be fitly 
suspended, but is filled with fluid with which 
the membranous labyrinth is every where sur- 
rounded, a provision not less necessary to the 
sense of hearing than is the fluid contained in 
the interior of the vestibule and semicircular 
canals. In all such Fishes, therefore, the 
auditory apparatus, consisting of a membra- 
nous vestibule and semicircular canals, is lodged 
on each side in cavities excavated in the base 
of the cranium and bounded by the temporal 
and lateral parts of the occipital bones. 
The internal ear itself (fig. 528) is composed 
of the following parts: ist. The membranous 
vestibule (fig. 525, i). 2d. The sac of the 
otolithe. 3d. The membranous semicircular 
canals. 
1003 
The membranous vestibule is an elongated 
smooth sacculus of very various form in diffe- 
rent Fishes. Its parietes consist of a pellucid 
membrane, and its outer surface is connected 
by loose cellular tissue to the sides of the 
cavity in which it is lodged. Its anterior 
extremity is somewhat dilated and contains 
a little otolithe; moreover into it open the 
ampulle of the anterior and external semi- 
Fig. 533. 
SSS 
Internal ear of Perch. (After Cuvier.) 
circular canals. The posterior extremity of the 
vestibule is narrower, and into this part opens 
the ampulla of the posterior semicircular canal 
and the hinder termination of the external one. 
Near the middle of the vestibular sac enters the 
wide duct formed by the conjunction of the 
terminations of the anterior and posterior semi- 
circular canals; but whether this wide duct 
ought rather to be looked upon as forming part 
of the vestibule or of the semicircular canals 
may be a matter of doubt, although the latter 
supposition is the most probable. 
Thus the six extremities of the three semi- 
circular canals communicate with the cavity of 
the membranous vestibule, not by six, but by 
five orifices. 
The membrane of which the vestibule con- 
sists is considerably thinner than that which 
forms the semicircular canals; indeed it is so 
delicate that if torn it at once collapses and is 
scarcely distinguishable from the surrounding 
arts. 
In the Pike ( Esox lucius) there is a re- 
markable appendage to the vestibule which is 
not met with in other Fishes. This consists of 
a pyriform membranous sacculus lodged in the 
commencement of the spinal canal, which opens 
into the vestibular cavity by a narrow orifice 
near the entrance of the posterior semicircular 
canal. The thickness of the walls of this 
sacculus is much greater than that of the pa- 
rietes of the vestibule, resembling rather in 
this respect the ampulle of the semicircular 
canals. Some of the upper spinal nerves are 
distributed to this organ, but they give off no 
branches, nor does it appear to receive any 
filament from the auditory nerve. 
The sac of the otolithe in most Fishes is 
immediately beneath and in close contact with 
the membranous vestibule, but in some it is 
hidden in the base of the occipital bone more 
remote from the vestibular cavity, with which it 
is joined by anarrower duct. The saccus is most 
generally divided into two portions by a median 
septum, in such a way, however, that the ante- 
