I ! „
5 6 THE COMMUNI C A T IO N OF SOUND, &c.
by the fame mediam to the bottom of the ear. Hence it is evident they have
no occaiion for a cavity of the tympanum nor for an Euftachian tube.
A t the bottom of the ear we find in all fiflies femicircular canals, fimilar in
lliape and fituation to thofe found in the mammalia, but much larger and more
extenfive in their furface ; in order, perhaps, to compenfate for the lefs forcible
impreflion made by the water in them than in us by the air.
Conneded with the femicircular canals, we find facs containing cretaceous
matter, which perhaps we may think have as much refemblance to the cochlea
as to the veftible in our ear, if we refled that in birds the place of the cochica
is fupplied by a fliort ftreight tube.
The cretaceous matter, which in the pifces'^f Linnaus, and fome of his
nantes pinnati, forms hard and rugged ftones, may ferve, like the fmall hard
bones of our ear, to convey a more forcible impreffion to the nerves fpread out
on the membranes which contain them.
Several of the cartilaginous fiihes, the raja, fqualus, &c. have a meatus
auditorius externus, through which the found is conveyed by a watery vifcid
liquor to the inner fides of the membranes of the femicircular canals and fac or
facs containing the cretaceous matter: but in the pifccs of Linnzeus, and in
fome of his nantes pinnati, as the fturgeon, I have not yet found a meatus
auditorius externus; and as I farther obferve, that the common canal or veftible,
where the three femicircular canals communicate, is feparated from the
cavity of the cranium by a thin membrane only; and that this cavity, in by far
the greater number of fiihes, contains a watery liquor in confiderable quantity;
and that, from the thinnefs of the cranium, the tremor excited by a fonorous
body may be as readily tranfmitted through their cranium to the water within
it, and fo to the ear, as through the temple of the turtle to his ear; I am more
inclined to fuppofe, that in them there is no meatus auditorius externus, than
that, from the want of accurate inve(ligation, it has not as yet been
difcovered.
C H A P .
II
O r THE E Y E S tK F ISHES, SI
C H A P T E R . XI .
O f the EYES in FISHES.
IN treating this fubjeft, I fliall firft make fome curfory obfemtions on the
coats, and afterwards confidcr the humours of the eye, their texture, fpe'
cific weight, fhape, and powers of refradion.
In fifiies, birds, and in all animals in which objefts often imprefs one eye
folely or chiefly, die optic nerve is, as i priori we might have fuppofed, much
more eccentrick than in man.
I have in a former work obferved (a), that the retina terminates in man near
to the roots of the ciUary proceffes; which circumftance is equally obfervable in
fiflies.
In all fiflies, fo far as I have obferved, the pigment on the inner fide of the
choroid coat is, as in land-animals which feek their food in the night-time, of a
bright colour at the bottom of the eye; perhaps becaufe the light ftnkes the
bottom of their eye witli lefs force than in the land-animals, many of its rays
being intercepted by the water. To account, however, for the diverfity of the
colour of tliis.pigment, or tapetum as it has been called, in the different genera
of animals, feems' to be a matter of much difficulty. Nay, it may be a queffion
whether the chief ufes of the choroid coat in any ammal hnve been clearly
afcertained ; or whether we certainly in what manner the choroid coat is
fubfervient to the retina. Perhaps attention to the powers of the eye m two
animals which are mere varieties of the fame fpecies, may ferve to throw farther
light on tliis curious fubjea-, I mean the brown and the white rabbit: for
in the former, the choroid is every where covered with a dark pigment; whereas
in the latter, although the choroid coat is as much compofed of veiTels as in
other animals, I have found tliat the black paint, tapemm, or inner layer of
the choroid, is altogether wanting: And hence the colour of the red blood circulating
in the veffels of the choroid is feen when we look into the eye, or makes
their eyes appear red f.
P Th e
(.,) On llie Nm-oxis Syll™, chap. xx!.
t I„ i.aico ,0 ¡«8»!»,,, Jilisint «.a..« of M' Wi»™ I di.k i. propc, o ottac, ,1.» I
I J ° . . . r - 1 A 1.:... :.. .-c. . In ...Ki.-li hp alli-Hui-H th.it
Icil t this
clioroi.i coat was altogctlicr wnming
nil cvi-or wliac Iw hml wrote to me. in the
wanting. It would be worth wliilc to c:
r.-ibbil.
circumihnce by a letter I received from Urn in fpviug 1781; in which he aUedgcd tl.at tiie
:he white rabbit. IN a fecomi letter, above fix months thereafter, he retr.iaed as
me I had found, that although they liave the choroid coat, the pigment is
l)ie ehwoid coat in the mixed breed between the brown and white