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