1004 
rior is much the larger compartment, to which 
the posterior chamber seems a superadded ap- 
pendix. Both of these compartments are filled 
with a pellucid fluid, and each contains a stony 
mass or otolithe, of which that in the anterior is 
the largest, that in the posterior being compara- 
tively of small dimensions. In Orthagoriscus, 
however, according to Cuvier, the saccus is 
single, and instead of an otolithe only con- 
tains a few granules apparently rather of 
mucus than of cretaceous substance. 
Otolithes—Most Fishes are furnished with 
three stony masses, which are intimately con- 
nected with the function of hearing. Of these, 
the otolithes or lapilli, one, generally the small- 
est, is contained in the anterior extremity of the 
vestibule; the other two are situated in the two 
compartments of the succus. The otolithe con- 
tained in the anterior compartment of the saccus 
is generally of remarkable size, forming a con- 
siderable protuberance in the base of the occi- 
pital bone, in which it is lodged ; this is con- 
spicuously seen in the Gadide and some of the 
erch tribe. 
The substance of these otolithes consists of 
carbonate of lime, but they assume various de- 
grees of hardness and considerable diversity of 
colour in different Fishes. In most cases they 
resent a texture as hard and fragile as porce- 
fain. In a few instances, as for example in the 
Sturgeon ( Accipenser Sturio ), there is only one 
lapillus, which is soft and as easily crushed 
and reduced to powder as a piece of chalk ; as 
is likewise the case with the otolithes of the 
Raide and Squalide. 
In shape the otolithes vary exceedingly in 
different genera. For the most part they are 
smooth and present this character in common, 
that they are marked with asperities, fossee, and 
grooves for the attachment or reception of nerv- 
ous filaments. Those contained in the saccus 
are frequently surrounded by a serrated margin, 
which is rarely the case with the lapilli of the 
vestibule. But whilst there is so much diver- 
sity in the shape of the otolithes belonging to 
different genera of Fishes, the form of those met 
with in the species belonging to the same genus 
is wonderfully constant, so much so, indeed, 
that not only the general outline, but the most 
minute fossules and grooves were found by 
Weber accurately to correspond in different 
specimens, so that it was difficult to distinguish 
one from the other; from which circumstance 
those otolithes might be employed with advan- 
tage as affording excellent generic characters to 
the zoologist. The connection of the otolithes 
with the saccus or with the vestibule is so 
difficult to be perceived, that they might be 
thought to be loose in the contained fluid ; 
when, however, we find them smallin the 
younger Fishes, and increasing in size as age 
advances, it is evident that they must receive nu- 
tritious vessels; they are moreover attached b 
nervous filaments of extreme delicacy, which 
pass to them from the saccus. In many points 
they touch the membranous walls of the cavity 
in which they are lodged; when, therefore, the 
sac is but loosely connected with the bones of 
the cranium, sonorous vibrations cannot be 
PISCES. 
communicated immediately from the cranium 
to the lapilli, but must first be communicated to 
the surrounding fluid. 
Semicircular canals—Al\ Fishes, with the 
exception of the Petromyzonide, have three 
se canals entering into i formation 
of the internal organ of hearing, and the arrange- 
ment of which is as follows. The anterior 
arises by one extremity from the anterior part of — 
the vestibule, and, winding upwards and back- 
wards, meets the rior semicircular canal | 
derived from the hinder part of the vestibular 
cavity ; at the point of meeting the two join to” 
form one common duct, which enters the vesti- 
bule near its middle. Both these canals are 
placed perpendicularly. The third or external — 
semicircular canal issues from the anterior ; 
the vestibule, and winds horizontally outwards to 
join the vestibule again at its posterior part near 
the origin of the posterior canal. In this way 
the three semicircular canals open into the mem-_ 
branous vestibule by pipe ec Tn the — 
ring, however, ( Clu arengus ) not 
the anterior and paaaaied canals ie, 
external also joins the posterior, so that in thi: 
fish there are only four apertures communicating 
with the vestibule. 
Each of the semicircular canals near its com= 
mencement from the vestibule swells into an 
oval dilatation called the ampulla, so that three 
of these ampullz exist, two at the anterior part 
of the vestibule, and the third near its posterior 
extremity, “ 
The connection between the semicirculat 
canals and the cranium is effected by the assist- 
ance of osseous passages, in which one or ts 
(rarely all three) of the semicircular canals are 
lodged, and in some Fishes, as for example in 
Cobitis fossilis, these are entirely deficient 
The membranous canals are not at all adherent 
to the osseous passages, but are only connected 
with them by the intervention of a most delicate 
cellulosity, or are merely suspended in a flui 
with which all the osseous canals as well as the 
entire cranium is filled up; they are conse. 
uently extracted without the employment 0 
the slightest force. * 
Those canals which are not enclosed in bon 
channels are simply annexed to the bones of th 
cranium by a fine cellular web. 
From the above arrangement it may & 
clearly understood that these parts are pul 
posely left but loosely connected to the surfa 
of the bones, for otherwise the bony cana 
would not so greatly exceed the membrane 
ones in size, but on the contrary would 
filled and lined by them throughout; and t 
sonorous vibrations most readily arrive at 1 
labyrinth through the fluid with which th 
canals are surrounded. A 
In Murena anguilla the anterior and pos 
rior semicircular canals mount so high towar 
the vertex of the cranium that they are 1 
placed by the side of the brain, but absolutel 
rise above it and approximate their fellows of 
the opposite side. = 
The length and calibre of the semicireulai 
canals vary very much, not only in different sp 
cies, but also when compared with each othe 
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