Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/204/
196 
EYE. 
presents greater curvatures does not require 
concave glasses, as the old person requires con¬ 
vex ones. The state of the eye, after the re¬ 
moval of the lens by operation for cataract, 
proves that it is a part of the organ essentially 
necessary for correct vision. When the eye is 
in other respects perfect, without any shred of 
opaque capsule,any irregularity or adhesion of 
the pupil, or any alteration in the curvature of 
the cornea, as in young persons who have had 
the lens properly broken up with a fine needle 
through the cornea, vision is so good for distant 
objects, that such persons are able to pursue 
their common occupations, and walk with safety 
through crowded streets, but they require the 
use of a convex lens, of from three and a half to 
five inches focus, for reading or vision of near ; 
old persons, however, generally require convex 
glasses on all occasions after the removal of the 
lens. That the curvatures of the lens are fre¬ 
quently different in different individuals may 
be inferred from the frequency of short sight, 
or defective power of adaptation, not attributa¬ 
ble to any peculiarity of the cornea. Petit 
states that he found lenses of which the two 
convexities were equal, and others of which the 
anterior was greater than the posterior, and 
more than once, one more convex on its ante¬ 
rior surface in one eye, while that in the other 
eye was in a natural state. He also occasion¬ 
ally found the lens as convex in the advanced 
period of life as in youth. I have repeatedly 
observed the perfection of vision and power of 
adaptation much greater in one eye than the 
other in the same individual, without any defect 
of the cornea, pupil, or retina ; and occasionally 
have found young persons requiring the com¬ 
mon convex glasses used by persons advanced 
in life, and old persons becoming near-sighted, 
and requiring concaves. The annexed letters 
shew the difference of curvature at the different 
periods of life, as represented by Sbmmerring. 
A is the lens of the fœtus; B, that of a child of 
six years of age ; and C, that of an adult. 
Fig. 117. 
A b_c 
r 
The colour of the lens is also different at 
different periods of life. In the fœtus it is 
often of a reddish colour ; at birth and in in¬ 
fancy it appears slightly opaque or opaline ; in 
youth it is perfectly transparent; and in the 
more advanced periods of life acquires a yel¬ 
lowish or amber tint. These varieties in colour 
are not visible, unless the lens be removed 
from the eye, until the colour becomes so deep 
in old age as to diminish the transparency, 
when it appears opaque or milky, or resembling 
the semitransparent horn used for lanterns. 1 he 
hard lenticular cataract of advanced life appears 
to be nothing more than the extreme of this 
change of colour, at least when extracted and 
placed on white paper it presents no other 
disorganization ; but the lens of old persons, 
when seen in a good light and with a dilated 
pupil, always appears more or less opaque, al¬ 
though vision remains perfect. The depth of 
colour is sometimes so great, without any 
milkiness or opacity, that the pupil appears 
quite transparent although vision is lost. This 
is perhaps the state of lens vaguely alluded to 
by authors under the name of black cataract. 
The consistence of the lens varies as much 
as its colour. In infancy it is soft and pulpy, 
in youth firmer, but still so soft that it may be 
crushed between the finger and thumb, and in 
old age becomes tough and firm. Hence it is 
that in the earlier periods of life cataracts may 
be broken up completely into a pulp, and 
absorbed with certainty, while in old persons % 
they adhere to the needle, unless very deli¬ 
cately touched, and are very liable to be de¬ 
tached from the capsule and thrown upon the 
iris, causing the destruction of the organ. On 
this account, therefore, the operation of extrac¬ 
tion must generally be resorted to in old per¬ 
sons labouring under this form of cataract, 
while the complete division of it with the 
needle and exposure of the fragments to the 
contact of the aqueous humour secures its 
removal by absorption in young persons. It 
must not, however, be forgotten that the softer 
lenticular cataract occasionally occurs in ad¬ 
vanced life. 
The crystalline lens is a little heavier than 
water. Porterfield, from the experiments of 
Bryan Robinson, infers that the specific gra¬ 
vity of the human lens is to that of the other- 
humours as eleven to ten, the latter being 
nearly the same as water; and Wintringham, 
from his experiments, concludes that the den¬ 
sity of the crystalline is to that of the vitreous 
humour in the ratio of nine to ten ; the spe¬ 
cific gravity of the latter being to water as 
10024 to 10000. The density of the lens is 
not the same throughout, the surface being 
nearly fluid, while the centre scarcely yields to 
the pressure of the finger and thumb, especially 
in advanced life. Wintringham found the spe¬ 
cific gravity of the centre of the lens of the ox 
to exceed that of the entire lens in the propor¬ 
tion of twenty-seven to twenty-six. The re¬ 
fractive power is consequently greater than that 
of the other humours. On this head Mr. 
Lloyd, in his Optics, says, “ In their refrac¬ 
tive power, the aqueous and vitreous humours 
differ very little from that of water. The re¬ 
fractive index of the aqueous humour is 1.337, 
and that of the vitreous humour 1.339; that of 
water being 1.336. The refractive power of 
the crystalline is greater, its mean refracting 
index being 1.384. The density of the crystal¬ 
line, however, is not uniform, but increases 
gradually from the outside to the centre. This 
increase of density serves to correct the aber¬ 
ration by increasing the convergence of the 
central rays more than that of the extreme parts 
of the pencil.” Dr. Brewster, in his Treatise 
on Optics, says, “ I have found the following 
to be the refractive powers of the different 
humours of the eye, the ray of light being 
incident upon them from the eye : aqueous 
humour 1.336; crystalline, surface 1.3767, 
centre 1.3990, mean 1.3839; vitreous humour 
1.3394. But as the rays refracted by the 
aqueous humour pass into the crystalline, and
        

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