On the structure of the eye of the swordfish (Xiphias gladius, Lin.)

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On the Structure of the Eye of the Swordfish (Xi-
phias gladius, Lin.)

By Robert Edmond Grant, M. D., F. R. S. E.,

F. L.S., M.W. S., &c.

Member of the Medico-Chirurgical Society of Edinburgh, Fellow of
the Royal College of Physicians of Edinburgh, &c.

From the Memoirs of the Wernerian Natural History Society.

F ROM the great density and obscurity of the medium
through which fishes receive the rays of light, compared
with those of the atmosphere, their organs of vision neces-
sarily present considerable differences in form and structure,
from those of land animals. Predatious fishes, which fre-
quent great depths, have very large eyes, from the general
darkness of the ocean, and probably from the diminished
energy of the nervous system in animals so low in the scale.
The eyes of the common swordfish, here described, were
four times as large as those of an ox, although the bulk of
the fish did not equal one half of the bulk of that quadru-
ped. Blainville has observed, that fishes which inhabit
the open sea have the eyes larger and better developed
than those which frequent the shore. As the refractile
power of the aqueous humour of the eye is the same as that
of water in which fishes, constantly reside, this humour is
unnecessary, and nearly absent, in that class of animals,

2

ON THE STRUCTURE OF THE EYE

and consequently their cornea is flat. This flatness of the
cornea, however, could not be preserved, if the eyes of
fishes were formed of soft membranes, like the eyes of
quadrupeds, as the lens’ would then be pressed forward by
the muscles surrounding the eyeball against the iris, and
completely prevent the pupil from changing its dimensions.
Hence we find a firm cartilaginous or osseous framework
in the eyes of fishes, to preserve the flatness of the cornea,
analogous to what is employed in birds to preserve a great
convexity of that part. The great thickness of the sclero-
tic coat, the presence of the choroid gland and adipose
matter between the sclerotic and choroid coats behind, and
the flatness of the cornea before, shorten very much the
axis of the eye in fishes. Hence a complete spherical form,
and greater hardness of the crystalline lens, is necessary in
these animals, to bring the rays of light more quickly to a
focus. Two small portions of a sphere would have been
sufficient, if the texture of the lens were homogeneous like
glass ; but this organ possesses a highly complicated struc-
ture, consisting of fibrous layers, beautifully disposed to
check the aberration of light, and increasing in density from
the surface to the centre. The distance between the cornea
and the retina in the swordfish is about one inch and a half,
while the transverse diameter of the eye is three inches. The
shortness of the axis, and the enormous size and spherical
form of the lens in the eyes of fishes, reduce greatly the pro-
portion of their vitreous humour. In the common herring,
the axis of the vitreous humour is only a fifth part of that of
the lens ; whereas in man, it is nearly four times greater than
the axis of the lens. In the large eye of the swordfish, the
axis of the vitreous humour is not one-half of that of the
lens. The ciliary processes and foramen centrale are not
found in fishes. The choroid gland, the solid sclerotic coat,
the fatty matter within it, and many other peculiarities in

or THE SWORDFISH.

3

the structure of the eye, belong exclusively to this class of
animals ; and besides many general peculiarities, the eye
presents in the individual species all the variety and irre-
gularity which characterize the organs of animals low in
the scale, and exhibits a gradual transition to the apparent-
ly anomalous eyes of molluscous animals. The broad flat
irregular form of the eye, and the quantity of matter inter-
posed between the outer coat and the retina, in the sword-
fish, closely resemble the appearances presented in the
eyes of the Octopus and Loligo. From the great size
and simple structure of this organ in fishes, a minute ex-
amination of these peculiarities might throw much light
on the nature of many parts, the uses of which are still
perfectly unknown in the higher orders of animals. The
eye of the common swordfish appears not to have been
hitherto examined ; and I am indebted for the opportunity
of making the following observations to Professor Jame-
son, who kindly presented me with the two recent eyes,
and the whole of the viscera, of the beautiful male specimen
of that rare animal, lately taken in the Frith of Forth, and
now preserved in the Museum of the University.

The eyes of the swordfish are so perfectly lateral in their
position, and so flat anteriorly, that that animal can per-
ceive objects only with one eye at a time. This is proba-
bly another reason, besides those stated above, for the eyes
of this fish being usually large. In the specimens I ex-
amined, the transverse diameter measured 3 inches, the
axis 2] inches, and the entire animal 7 feet 2 inches from
the point of the sword to a line drawn between the tips of
the tail. The general form of the eye is that of an irregu-
lar hemisphere, a little depressed in the direction of the
axis, the anterior part corresponding to the flat base of the
hemisphere. The eye is circular round the axis, but its
sphericity on the back part is very imperfect from the irre-

4

ON THE STRUCTURE OF THE EYE

gular form of the osseous plate of1 the sclerotic coat. The
sphericity of the retina within is not interrupted by this ir-
regular form of the external coat, as there are several soft
parts interposed between them. The conjunctiva is black,
like the skin of the animal, to the margin of the transparent
cornea. The pupil is irregularly circular, as in many other
fishes. The iris has a greenish-white colour, and a beauti-
ful metallic lustre. The cornea is flat, and about lj inch
in diameter. The anterior edge of the osseous plate pro-
jects beyond the margin of the cornea, so as to make the
anterior part of the eye a little concave externally. There
is no eyelid, but a looseness of the conjunctiva round the
edge of the orbit, to admit of the motions of the eye. The
conjunctiva does not pass loosely over the cornea, as in the
haddock and many other fishes, but adheres so strongly as
only to be separated from it with difficulty, after long ma-
ceration. The muscles of the eyeball are very strong and
red coloured, like those of a quadruped, and correspond
with the extensive distribution of the red globules of the
blood through the other parts * of this animal. The adipose
substance, in which the eye is imbedded, is very different
from the soft white watery substance we usually find in this
situation in smaller fishes. It is firm, of a yellowish-white
colour, and filled with an oily matter, which agrees much
in its smell, colour, and other properties, with the oil pro-
cured from the whale.

The tunica conjunctiva can be separated with ease from
the surface of the sclerotic as far as the margin of the cor-
nea, where it is firmly connected with the projecting edge
of the osseous plate. After long maceration, it can be torn

* See my account of the anatom}r of the thoracic and abdominal vis-
cera of the Swordfish, in the Transactions of the Medico-Chirurgical
Society of Edinburgh, vol. iii.

OF THE SWORDFISH.

5

from the whole surface of the cornea, with which it appears
to be connected by cellular substance. But, by the same
treatment, the cornea can as easily be separated into many
layers, as into the two formed by tearing off the conjunc-
tiva. The cornea is very thin, compact, and of equal thick-
ness throughout ; it is lodged in a small groove of the os-
seous plate ; but, at the nasal side of the eye, it passes a
little over the inclined margin of that plate. The outer
layer of the sclerotic coat consists of strong white shining
tendinous fibres, forming a compact membrane, which ad-
heres firmly to the whole outer surface of the osseous plate.
The fibres of the outer covering of the optic nerve, which
is considered to be a continuation of the dura mater, are
very distinctly seen to mix and disappear among the
fibres of the sclerotic, so as to form with it a continuous
membrane. This layer of the sclerotic is connected,
like the conjunctiva, to the outer margin of the cornea.
The osseous plate of the sclerotic covers the whole of the
eyeball, excepting the aperture on the fore part covered by
the cornea, and a space nearly of the same size on the back
part of the eye. The irregular round opening on the fore
part of the plate is 1 1 inch in diameter, with an inflected
margin 3 lines in thickness. The plate is hard, translucent,
somewhat brittle, of a yellowish-white colour, and strongly
marked fibrous texture. It is thinnest and most pellucid
where its diameter is greatest, and it terminates posteriorly
by a thin and very uneven margin. It does not effervesce
in nitric acid, like substances containing carbonate of lime,
and appears to be of a condensed horny texture, containing
a quantity of oily matter between its fibres. It is divided
into two equal parts, an upper and a lower, by an uneven
suture, passing from before backward, or in the direction
of the axis. The tendinous extremities of the straight and
oblique muscles of the eye pass through the outer layer of

6

ON TIIE STRUCTURE OF THE EYE

the sclerotic, and are firmly inserted into the osseous plate.
On the upper half of the plate, about half an inch from its
ragged posterior margin, there is a perforation about two
lines in diameter, for the passage of the arteries and veins
of the eyeball. On the inside of this perforation there is a
bony ridge, about a line in thickness, for the protection of
the vessels passing through it. This hard inner layer of
the sclerotic is continued in a membranous form, and in
firm union with the outer layer, from the ragged posterior
margin, to the optic nerve. The broad anterior inflected
margin of the plate affords attachment to the iris and the
cornea, and gives lodgment to a white adipose thick band
surrounding the eye, and placed anteriorly to the ciliary
ligament. Immediately within the osseous plate we ob-
serve a very delicate black coloured membrane, adhering
loosely to it as far as the ciliary ligament, where it becomes
inseparably united with the vascular layer of the choroid.
This thin membrane lies immediately over and in contact
with the fatty substance enveloping the choroid gland, and
occupies the situation of the white silvery pigment which
forms the outer layer of the choroid coat in many osseous
fishes. Near the ciliary ligament this black membrane ac-
quires a light grey colour and metallic lustre ; but at the cili-
ary ligament, where it unites inseparably with the vascular
layer of the choroid for a line and a' half, it again becomes
dull, and has a deep purple colour. On the fore part of
the ciliary ligament it resumes its metallic lustre, becomes
detached from the choroid, and can be traced with ease
dipping into an angular recess between the uvea and the
iris, and again mounting to descend over the fore part of
the iris. In this animal there is a wide angular space, a
line in breadth, between the uvea and iris, which contains
a white cellular substance. The iris is thick and firm, like
cartilage, and, on removing its metallic pigment from the

OF THE SWORDFISH.

7

fore part, it presents a black surface. The white adipose
band surrounding the eye before the ciliary ligament, rests
on a peculiar thin membrane, which separates it from the
outer dark layer of the choroid, and extends forward as far
as the cornea. The dark coloured external layer of the cho-
roid, in the Xiphias, somesvhat resembles the thin black
lining of the sclerotic of quadrupeds, which was thought
to be a continuation of the pia mater. There is no silvery
pigment in this animal covering the choroid gland, which
we find so abundant in many osseous fishes, as the cod and
haddock.

Between the thin dark outer layer of the choroid and its
thick vascular layer, there is an immense deposit, of a wliite
semifluid fatty substance, half an inch in thickness, on the
back part of the eye, and extending as far forward as the
ciliary ligament. The fatty deposit is thickest on the back
part, near the optic nerve, and becomes gradually thinner
towards its anterior termination ; it is traversed in every part
by numerous large branches from the ocular artery, and by
the ciliary veins returning to the choroid gland. These
vessels are filled with red blood, and are so numerous as to
give a red tinge to the oily substance through which they
are ramified. The great bed of fat completely envelopes
that singular organ the choroid gland ; it enables the great
trunks of the ciliary arteries to subdivide, before penetrat-
ing the choroid, to form its innermost layer ; and it enables
the internal parts of the eye to assume a sphericity which
the osseous plate of the sclerotic is far from possessing.
The choroid gland has the same deep blood-red colour,
firm consistence, and vascular texture, as in other fishes ;
it is composed of two parts surrounding the optic nerve,
the one shaped like a horse-shoe, and the other nearly
straight. These two pieces are flat, about half an inch
broad, and nearly a quarter of an inch thick. The venous

8

ON THE STRUCTURE OF THE EYE

sinus, described by Blainville (Anat. Comp. t. i. p. 425)
as occurring in the inner part of this singular fleshy mass,
is large and distinct in the Swordfish, where it appears like
the pelvis of a kidney, with numerous openings, probably
of the ciliary veins, terminating in it. Blainvili.e consi-
ders the choroid gland of fishes as a vascular ganglion, or
sinus, for the reception of the ciliary veins. I may observe,
that, on cutting open this body, in the Swordfish, we find
its outer part composed of a compact parenchymatous tex-
ture, and, as we approach itsunner sinus or pelvis, it con-
sists of distinct straight parallel fibres, whieh may possibly
be ducts or vessels, leading to the central cavity . After
remaining long in spirits, the inner concave margin of the
organ splits spontaneously, and always in the direction of
these parallel fibres. The vascular membrane of the choroid,
which lies immediately beneath the bed of fat, is distinctly
separable, in the Swordfish, into two layers. The outer,
or venous layer, has a bluish-grey colour, and a strongly
marked corded or striated appearance, produced by the
straight veins which compose it. The inner, or arterial
layer, termed the Tunica Ruyshiana, and composed of the
ramifications of the ciliary arteries, has a deep brownish-
purple colour on the surface where the pigment rested.
At the place of the ciliary ligament, this inner layer of
the choroid assumes a yellowish-white colour, and conti-
nues of this light colour over the whole of the uvea to
its free margin, where it unites at an acute angle with
the iris. This white part of the choroid, and the white
surface of the uvea, are covered, in the natural state, like
every other part of the choroid, with a thick layei of a
brownish-black pigment. There are no ciliary processes
projecting in towards the margin of the lens, but in their
place we find the choroid distended by the white adipose
band I have already mentioned as surrounding the fore

OF THE SWORDFISH.

9

part of the eye, on the outside of the choroid, and near the
margin of the uvea. Blainville thinks he has observed
ciliary processes in the eye of the Squcilus mcixivius. At
the place where the optic nerve penetrates the inner layers
of the choroid, it has a very contracted and strangled ap-
pearance ; and the retina which it forms, is proportionally
the most bulky I have yet met with in any animal. The
retina forms a thick pulpy coat, of a bluish-white colour,
and becomes more thick and opaque as it advances towards
the uvea, near the base of which it terminates by a smooth
and even margin. We observe a straight white line or fissure
extending, on the nasal side of the eye, in this animal, from
the entrance of the optic nerve to the base of the uvea,
along the inside of the choroid coat. Along the whole of
this line the retina is firmly bound to the choroid coat, and-
it has no connection with the choroid at any other part.
This remarkable appearance has the closest resemblance to
the long fissure of the choroid through which the retina
enters in the class of birds, but is very rarely met with in
fishes. The eyes of the cod, the haddock, and many other
osseous fishes, present no appearance of this kind, but have
the retina quite free round the optic nerve. The retina
appears to form a larger mass tha,n could result from the
mere expanded filaments of the contracted part of the optic
nerve. The pulpy external layer of the retina is so thick
in the Swordfish, particularly near its anterior termination,
that it can be torn with the forceps, like cellular substance,
into several layers. The white membrane or ligament, cor-
responding to the pecten of birds, already discovered by
Blainville, in six different genera of fishes, is strong and
distinct in the Swordfish, proceeding from the anterior end
of the fissure of the choroid, through which the retina en-
ters, to the capsule of the lens. Although the lens is a so-
lid sphere, an inch in diameter, the pecten is so firmly at-

10 STRUCTURE OF THE EYE OF THE SWORDFISH.

tached to its capsule, as to bear its weight with ease when
suspended out of the water. On the opposite side of the
lens there is a smaller ligament of the same kind. The
great body of the lens, inclosed in a capsule of almost
tendinous consistence, after remaining some weeks in spi-
rits, splits spontaneously into concentric lamellae, and deep
rents in the direction of its axis, and separates easily into
coarse fibres having for the most part the same direction.

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