EYE.
197
those from the crystalline into the vitreous
humour, the indices of refraction of the sepa¬
rating surface of these humours will be, from
the aqueous humour to the outer coat of the
crystalline 1.0466, from the aqueous humour
to the crystalline, using the mean index, 1.0353,
from the vitreous to the outer coat of the cry¬
stalline 1.0445, from the vitreous to the crystal¬
line, using the mean index, 1.0332.” Dr.
Young says, “ On the whole it is probable
that the refractive power of the centre of the
human crystalline, in its living state, is to that
of water nearly as 18 to 7; that the water im¬
bibed after death reduces it to the ratio of 21 to
20 ; but that on account of the unequable den¬
sity, its effect in the eye is equivalent to a
refraction of 14 to 13 for its whole size.”
Respecting the chemical composition of the
lens, Berzelius observes, that “ the liquid in
its cells is more concentrated than any other
in the body. It is completely diaphanous and
colourless, holding in solution a particular
animal matter belonging evidently to the class
of albuminous substances, but differing from
fibrine in not coagulating spontaneously, and
from albumen, inasmuch as the concentrated
solution, instead of becoming a coherent mass
on the application of heat, becomes granulated
exactly as the colouring matter of the blood
when coagulated, from which it only differs in
the absence of colour. All those chemical
properties are the same as those of the co¬
louring matter of the blood. The following
are the principles of which the lens is com¬
posed : peculiar coagulable albuminous matter
35.9, alcoholic extract with salts 2.4, watery
extract with traces of salts 1.3, membrane form¬
ing the cells 2.4, water 58.0.
From the preceding observations it might
reasonably be supposed that the lens is com¬
posed of a homogeneous material, such as al¬
bumen or gelatine, more consolidated in the
centre than at the circumference; but this is
not the case; on the contrary, it exhibits as
much of elaborate organization as any other
structure in the animal economy. It consists
of an outer case or capsule, so totally different
from the solid body contained within it, that
they must be separately investigated and de¬
scribed. The body of the lens, it has been
already stated, consists of certain saline and
animal ingredients combined with more than
their weight of water, and when perfectly
transparent presents the appearance of a tena¬
cious unorganized mass; but when rendered
opaque by disease, loss of vitality, heat, or im¬
mersion in certain fluids, its intimate structure
becomes visible. If the lens with the capsule
attached to the hyaloid membrane be removed
from the eye and placed in water, the following
day it is found slightly opaque or opaline, and
split into several portions by fissures extending
from the centre to the circumference, as seen
in fig. 118. This appearance is rendered
still more obvious by immersion in spirit, or
the addition of a few drops of acid to the
water. If a lens thus circumstanced be al¬
lowed to remain some days in water, it con¬
tinues to expand and unfold itself, and if
delicately touched and opened by the point of
a needle, and carefully transferred to spirit,
and as it hardens is still more unravelled by
dissection, it ultimately presents a remarkable
fibrous or tufted appearance, as represented in
the figure below, drawn by me some years ago
from a preparation of the lens of a fish thus
treated (the Lophius piscatorius). The three
annexed figures represent the structure of the
lens above alluded to : A is the human crystal¬
line in its natural state; B, the same split up into
its component plates ; and C, unravelled in
the fish.
Fig. 118.
C
A B
This very remarkable structure of the body
of the lens appears to have been first accu¬
rately described by Leeuwenhoek, subse¬
quently by Dr. Young, and still more recently
by Sir David Brewster. Leeuwenhoek says,
“ It may be compared to a small globe or
sphere, made up of thin pieces of paper laid
one on another, and supposing each paper to
be composed of particles or lines placed some¬
what in the position of the meridian lines on a
globe, extending from one pole to the other.”
Again he says, “ With regard to the before-
mentioned scales or coats, I found them so
exceedingly thin, that, measuring them by my
eye, I must say that there were more than two
thousand of them lying one upon another.”
“ And, lastly, I saw that each of these coats
or scales was formed of filaments or threads
placed in regular order, side by side, each coat
being the thickness of one such filament.” The
peculiar arrangement of these fibres he describes
as follows : “ Hence we may collect how ex¬
cessively thin these filaments are; and we shall
be struck with admiration in viewing the won¬
derful manner they take their course, not in a
regular circle round the ball of the crystalline
humour, as I first thought, but by three dif¬
ferent circuits proceeding from the point L,
which point I will call their axis or centre.
They do not on the other side of the sphere
approach each other in a centre like this at L,
but return in a short or sudden turn or bend,
where they are the shortest, so that the filaments
of which each coat is composed have not in reality
any termination or end. To explain this more
particularly, the shortest filaments, M K, H N,
and O F, which fill the space on the other
side of the sphere, constitute a kind of axis or
centre, similar to this at L, so that the fila¬
ments M K, having gone their extent, and filled
up the space on the other side, in like manner
as is here shewn by the lines ELI, return
back and become the shortest filaments II N.
These filaments H N, passing on the other side