Page:The fairy tales of science.djvu/132

104 The human eyeball is a globular mass, somewhat flattened in front, and about the size of a walnut. The white part surrounding the centre is called the sclerotic coat, deriving its name from a Greek word expressive of hardness. This white coat is continued round the back of the eyeball, and forms a sort of strong bag for containing the other parts of the eye. As it is perfectly opaque, it is not continued over the front of the eye, but joins the beautiful transparent membrane called the cornea, or horny coat, which bulges forward a little, and forms that wonderful bow- window through which the rays of light pass to the brain. Within or behind the cornea may be perceived the iris, a sort of coloured fringe which assumes different hues in different eyes, being dark brown, blue, hazel, or grey, and, in exceptional cases, red. When we speak of blue eyes or hazel eyes, we refer to the colour of this remarkable fringe or curtain. In the centre of the eye, surrounded by the iris, is a dark circular space of variable dimensions, called the pupil, which is in fact the opening through which light passes into the dark chamber of the eye.

The internal structure of this wonderful organ is very complicated. The hard white membrane is lined by a coat called the choroid, which is covered on the inside with a perfectly black pigment, and this again with a delicate network of nerves called the retina. The cavity surrounded by these coats is filled by three substances, called humours.