a globular substance is concealed behind the eyelids. This is the dark chamber, the structure of which we are going to examine. It is composed of three coats or coverings, and three humours, or transparent substances, which fill the interior of the globe.

We will first speak of the coats that compose the walls of the chamber.

The outer coat is a very tough membrane; it is also hard, and from this property derives its name, sclerotica, a Greek word, expressive of hardness. It protects and supports the tender parts within it.

In the section you may suppose the upper eyelid to be situated at E, the lower at B, the opaque sclerotica surrounds the back of the eye, except where it is penetrated by the optic nerve at O, and is represented by the outer line, marked CCC: we may compare it to the shell of a walnut.

That part in front of the eye is called the cornea, from cornu, the Latin word for horn, which you know is a hard, transparent substance.

The cornea is the window of the eye, a bow, window; you see it bulges forward, projecting beyond the line that would have been formed by the regular continuation of the sclerotica :* we shall see the advantage of this presently.

Before we proceed any farther, I will give you some slips of card, on which the Greek and Latin names of the different parts of the eye are written, with the meaning of each affixed to it. If you at any time forget the explanation, you may then refer to your cards.

COATS OF THE EYE.

SCLEROTICA;-the hard outer coat.

CORNEA;-from horn, hard and transparent.
CHOROID ;-containing a likeness.

PIGMENTUM NIGRUM ;-black paint.

RETINA;-from rete, a net.

HUMOURS OF THE EYE.

VITREOUS; or glassy humour.

CRYSLALLINE LENS.

AQEOUS ;-or watery humour.

Arnott, ib. 240.

CHOROID.PIGMENTUM NIGRUM. 187

Having observed the hard, outer wall of the chamber, we now come to the second coat of the eye. This is a finer membrane, or web, lining the hollow of the opaque sclerotica; it is called the choroid, from two Greek words, signifying, contain and likeness. The choroid contains, within the cavity which it lines, a likeness, or image, of the objects presented to the eye. Over the choroid is spread a dark pigment, adhering loosely to its hollow surface: it is called pigmentum nigrum ; i. e. black paint: the use of it is to darken the chamber of the eye. This room would appear very dark and gloomy, if the walls, ceiling, and floor were painted black; but in the eye no light is useful, except that which falls upon the image, it is necessary that it should be a dark chamber.

I will take the sliding part quite out of the camera-obscura, and you will then see that the same expedient has been resorted to, in order to absorb or destroy the enter through the lens.

superfluous rays which Black has not the pro

perty of reflecting light.

Here is a striking

188

PUPIL OF THE EYE.

resemblance, a provision made expressly for the same purpose in the eye and in the camera.

The choroid is represented by the line next to the sclerotica: we do not see it in the living eye: it is a lining extending only to the edge of the cornea, the bow-window in front, where it adheres to a groove in the sclerotica.*

I shall not, at present, notice that beautiful part of the eye called the iris, which is very dark, or blue, or hazel, or grey, in different persons. There is no part corresponding to the iris in the camera or the telescope: we will therefore speak of it afterwards.

The next object of our attention is the pupil of the eye, which appears like a black spot in the centre of the iris; but it is, in fact, the opening through which light enters the dark chamber of the eye. It corresponds with this round hole in the sliding part of the camera-obscura; and, as in the camera, there is, immediately behind that opening, a double convex lens, so also, in the eye, behind the pupil, a double convex and

*Chambers's Dict. Blumenbach, El. Physiol. 247.