^iiill'
é f - i i '
fer,:
" f ;
il
t
5 S O r THE E Y E S IN F I S H E S .
The iris, though not the continuation of the choroid coat, rcfembles it in
being covcred on its inner or back part by a daric pigment; but it has the addition
of a coloured matter laid over its fore part; and thefe concealing its veffels
and their contents in a living animal, feveral eminent authors, whom Dr Haller
follows, have entertained the erroneous idea, that the arteries of the iris in man
and quadrupeds do not in life convey the red part of the blood, but the lymph
alone; and that they are eolourlefs or lymphatic (i). Yet thofc very authors
admit, that the arteries of the iris may be filled with an injedion coloured with
the powder of vermihon; and Dr Haller in particular has delineated the large
branches of thofe arteries in a very juft and elegant manner as they appear to
the naked eye (.). By what power or mechanifm, then, could he, Mr Ferrein
and others, conceive the red blood to be excluded in life ?
The plain facts here are, as I have repeatedly obferved, That by a fkilful
injeaion the whole iris, to the very edge of the pupil, may be made nearly as
red as a piece of fcarlet cloth, ^dfy. That the whole courfe of the blood from
the trunk of the ocular artery to the arteries of the iris, may be traced with the
naked eye. ¡d/j. That if any part of the iris be cut or pundured in a man
which I have feen happen in the attempt of extracling the cryftalline lens, red
blood immediately flows from its veffels. Nay, in one fueh cafe, a redcoloured
fungus, rooted in the iris, afterwards puflied out at the wound in the
cornea.
In one Und of fiil, the Ikate, the upper part of the iris in the horizontal
pofturc IS formed into a beautiful palmated procefs (d) ; which it is probable the
anirnal has the power of elevating and deprefling according to the quantity of
h g h t ; and perhaps he lets fall this curtain in time of ileep. It may likewifl be
worth while to obferve. whether the mind of the animal, independent of the
degree of hght poiTeiTes fuch a power over it as I have very long ago obferved
to be exerciTed in an obfcure light/ opens and lliuts the
pupil alternately when
In man the cornea is, beyond doubt, covered with the continuation of the
ikm; which m the found ftate poiTeiTes a confiderable degree of fenfibility and
when irritated by extraneous fubftances flicking in it, occaiions, as I have feveral
times feen, violent inflammation of the whole eye; in confequenee of which
fpeeks full of vefTels containing red blood, are often produced upon the corneaand,
what is remarkable, and ufeful to be attended to by the ocuHft thefe vef
fels, from whatever caufe produced, are, fo far ^ I have obferved, not the original
veiTels of the cornea running between its' layers, dilated by the inflamma
tion but are new veffels growing from the veffels of the tunica adnata, and extended
over the external f.rface of the cornea. Hence we perceive the propriety
of attempting to remove thefe by operation and external applications {.)
The
W Ruyfch, VieuiTcn», Fcrrdn, Mc«,. d. I'Ac. . 735. H^,,. p. ^^^^
(0 Haller, Fafdcul. VII. Tib. VI. fig. 4.5. c.
^ w Dr El. Phyf, L. x,!., p. „njufll,- d.nk. fcnfibilir, of the corn..
(//) See Tab, VII. fig, j.
O F .THE È Y E S F I S ri Ë S.
The inner part of the cornea feems to be lined with a thin very denfe membrane,
fit for confining the aqueous humour. But in man and quadrupeds it is
very difficult to feparate thefe coats of the cornea from each other; whereas in
fiflies they may be feparated with tlie utmoft eafe.
The humours of the eyes of fiihes are proportionally in greater quantity, or
much larger, than thofe of animals living in air. Thus the eye of the cod is
very nearly of the fame weight and depth, and its axis of the fame length (jQ,
as the eye of the ox. On this account I have, in the following experiments,
made chiefly the comparifon of thofe with each othen
As the rays of light fufler great refraflion in paffing from the rare medium of
uir into water, the cornea of land-animals is very convex. From this they poffefs
alfo two other advantages ; that a fmall motion of their iris excludes the
fupcrfluous quantity of light ; and that their cryftalline lens needs not be very
convex, and may therefore be placed at a diftance from danger.
But as there is little or perhaps no refradion of the Hght paffmg from faitwater
into the aqueous humour of the eyes of fiflies, their cornea has little convexity,
and perhaps would have been altogether flat, but that a quantity of
aqueous humour is neceffary for giving room and protection to their very convex
cryflalline lens, and for allowing the motiop of the iris. Nay, in one animal,
wliich makes the neareft approach to this clafs of animals, the fepia loligo,
I have found the cryftalline lens attached to the cornea, and the aqueous humour
and iris altogether wanting
The cryflalline lens is known to be compofed of concentric layers, and thefe
of fibres ; and the central part being harder than the furface of the lens, has
been called its nucleus. But it may be obferved, that the hardnefs does not
increafe from the furface to the centre in a perfectly equable manner; but that,
at the depth of one third nearly of the diftance of the furface from th.e centre,
there is a fudden increafe of the hardnefs of the lens : Sn that if the capfulc of
the lens be feparated from a nuinher ol" lenfes of the fame kind of animal, and
the lenfes rolled between the thumb and fingers in a bafon-full of water, with
the view of detaching the foft fuperficial parts, the nuclei of all the lenfes will
remain neai-ly of one fize ; and their diameter will be to the diameter of tiie entire
lens nearly as two to three.
After repeatedly comparing together the fpecific gravity of the aqueous, the
cryftalline, and vitreous humours of the ox and cod, by weighing them in aif
and water, I found their proportional weight to be as follows :
Spring-water,
Aqueous humour,
The vitreous humour of the ox,
The vitroous humour of the cod,
Parts.
1000
. 1000
1016
IOI3
The
Lc in fuppofing tlic eyes cf fifhei lo be flatter behind, as «ell as before, than the eyes ef
J: I