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296 THE NORTHERN MICROSCOPIST. 
the crystalline lens. The real mechanism of accommodation has 
been much disputed, but the results, as observed, are, that the cur- 
vatures of the crystalline lens are altered as the observer adapts 
his eye to near or remote vision ; increase of curvature, of course, 
shortening the focal length of the crystalline lens, and being better 
adapted for near vision, while the shallower curve is necessary for 
the distant view of remote objects. Helmholtz has shown that the 
radius of curvature of the anterior surface of the crystalline lens 
may be varied by means of the muscular arrangement, from 6 to 10 
millimetres. 
We may now cast another glance 
at the iris. ‘This apparatus is really 
a continuation of the choroid tunic 
which lies between the sclerotica 
and the retina: it ends in front, in 
what are called ciliary processes 
/ which you may see in the picture 
on the screen. The small muscular 
ring surrounding the pupil is called 
the sphincter muscle. (Fig. 24.) 
Now, the principal use of the 
choroid tunic, or rather the pig- 
mentum nigrum which it contains, 
Fig. 24. is to absorb those rays of light 
which have passed through the transparent retina, preventing their 
reflection, which would interfere with the distinctness of the image. 
I can now show you a dia- 
gram representing the pos- 
terior inside of the eye, after 
Henle. You will see the 
choroid tunic, the retina, and 
sclerotica as the three outside 
tings, while the centre is rami- 
fied by nerve filaments and 
blood-vessels. (Fig. 25.) 
These nerve filaments and 
blood-vessels lie in the retina, 
which really forms a continua- 
tion and extension of the 
optic nerve; it touches the 
outer circumference of the 
Pig. 25. iris at the front, and lies open 
as a cup-shaped disc in the interior of the eye, it receives the 
rays of light which have passed in turn through the cornea, aqueous 
humour, crystalline lens, and vitreous humour, and forms a 
picture at the focus of these. 



