The science of life : fully illustrated in tone and line and including many diagrams

THE HARMONY AND DIRECTION OF THE BODY-MACHINE

Lens Cornea

Iris

ese

Yellow Spot

Nerve

Fig. 47. The eyeball cut across to show its essential parts.

their homologues in other mammals. But now, in dealing with sight, we come into

our own. Man and ape and monkey rely on vision far more extensively than any other mammals, and it is probable from its minute structure that the human eye can see more distinctly than any other, save that of some birds. Here, then, is our consolation. We have such excellent eyes that we have no need of noses, and we may interpret the enviably sensitive canine muzzle as a sign of the fogginess of the canine eye.

The structure of an eye is very like that of a photographic camera. We may distinguish two essential parts; a sensitive screen at the back, the retina, and an optical system that projects an image of the outside world on to that screen. The retina is the sense-organ proper, for it is here that the rays of light forming the image act on sensitive cells and initiate nervous impulses. The eyeball is blackened within, like a camera, in order ‘9 prevent reflection and scattering of light. The structures in the front part of the eye are very strikingly like the structures in the front part of a camera. The wall of the eyeball in this region is transparent and bulges forwards as the cornea ;

a short distance behind the cornea there lies a doubly convex mass of transparent tissue, the lens. The iris of the eye works like the camera’s diaphragm. When illumination is poor

dusk—the iris opens widely, so that as much light as possible enters the eye ; when the light is very bright it closes down to a pin-point, for if the intensity of light inside the eyeball exceeds a certain value the retinal cells are injured and become temporarily or permanently incapacitated. There is another advantage to the use of an iris diaphragm, whether in the eye or in a camera, depending on the fact that the middle of a lens always focuses light more accurately than the edges. If the light is bright enough it is always a good thing to stop down the iris to a small size and cut out the edges of the lens, so that the image formed has the least possible distortion of colour and line. The lens divides the cavity of the eyeball into two parts. Of these, the part in front of the lens is filled with a watery fluid, the aqueous humour, and that behind it is filled with a transparent jelly, the vitreous

eee: Muscles —in a dark room, for example, or at x

humour. ‘The aqueous humour is constantly being secreted by glands in the ciliary region

Tear gland

duct

Fig. 48. Why we sniff when we cry.

The gland that makes tear-fluid to keep the eye clean and the duct

which drains it away into the nose.