SIGHT. i, 
Sight. 
The organ of sight is the Eye, which is a spherical body, and com- 
posed of several protective coats on the outside, and of a delicate optical 
instrument in the inside. The outer wall of the eyeball is composed of 
three coats. The outermost coat, called the selerotic,! 1, fig. 60, is a strong, 
tough, fibrousmembrane, 
the use of which is to 
protect the parts inside. 
It does not cover the 
whole of the eye; the 
front of the eye, or that 
part which is visible, is 
covered with a trans- 
parent, horny or cartila- 
ginous plate, called the 
corned,” 2, which fits into 
the edge of the sclerotic, 
almost exactly in the 
way the glass of a watch 
fits into the case. The 
cornea, as may be seen from the figure, bulges out a little beyond the 
line of the sclerotic, or is rather more convex. The second coat is the 
choroid,’ 3, a more delicate structure than the last, consisting almost entirely 
of blood-vessels and nerves. It is of a very dark colour; and it is this 
dark coat, seen through the narrow opening in the centre of the iris, 
which causes the round black spot to appear in the centre of the eye. 
Like the sclerotic, this coat also gives place to another in front of the 
eye; instead of the choroid, in front is the cris,‘ 6, 6, a coloured mem- 
brane, which hangs down behind the cornea. The use of the iris is to 
act as a curtain, to regulate the amount of light that is to be admitted 
into the eye; for it is contractile, and can thus diminish the size of the 
little hole, or pupil,®> 7, when the light is too strong, as may be observed 
in the eye of any one looking at a strong light. The third coat is the 
retina,® 8, so called because it is a complete network of nerve-fibres from 
the optic nerve, 15, 16, extending all round the hack part of the eye, 
being deficient towards the front. 
In the interior of the eye, we come first to the aqueous humour,’ 
> 
which is a fluid little else than pure water, filling the anterior chamber 




1 From Greek skleros, hard, stiff. 2 From Latin cornu, a horn. 
3 From Greek chdrion, any skin. 4 From Greek its, a rainbow. 
5 From Latin pupillus, a child; because, on looking into it, one sees one’s own image 
reflected exceedingly small. 6 From Latin rete, a net. 
7 From Latin agua, water; humour is any fluid or moisture, from Latin Aumeo, to be moist. 
