STRUCTURE AND FUNCTION OF THE EYES

"nature is not stingy. She has not given the human race a meagre inheritance. She -did not blunder when she made the human body, nor did she allow the spirit of man to develop a civilisation to whose demand his body is not equal. . . . Her motto seems everywhere to be, 'Provide for the emergency, enough and to spare! ". J. A. JACKSON, M.D.

THE eye is one of the most intricate and fascinating of our organs because it is so intimately and directly connected with the brain. Embryologists state that one of the earliest developments of foetal life is the protrusion of the optic, nerve from the brain. As the eye-develops in the embryo, it remains connected with the brain by the optic nerve which expands within the eye into a delicate membranous innermost dining, the retina. This retina, located d at the' back of the eye, is the sensitive plate upon which, in the normal eye, the image falls, awaiting interpretation by the brain, in the same way that the picture is registered on the film of a camera until the owner develops it.

In the normal eye the retina consists of ten layers. The ninth or next to the innermost and most important .contains the cone- and rod-shaped nerve ends which are essential to the function of seeing. The cones, which are to distinguish colours, are concentrated toward the - centre of the 'retina; the rods, which are presumed to aid in night vision, toward its outer edges. Situated in layer of the retina, just a bit off-centre is a small yellow spot called the macula lutes, or centre of sight. Here vision is keenest. The macula is almost entirely composed of cones. In the heart of the macula is a small depression called the fovea centralised, or central , a still more sensitive group of cone-shaped nerves whose duty it is to hunt light.

It is the macula that presents a clear picture to the mind for interpretation. The other retinal nerves take less distinct images can notice for yourself the difference between vision in the centre of sight and in the less sensitive retinal nerves surrounding it. Try this experiment: Hold your opened hands beside your ears six inches from the head, palms forward and parallel with the shoulders. Keep your eyes straight ahead on a definite object such as a door knob. Now, wiggle your fingers. With the eyes straight ahead, you see something on each side of your face but if you did moving not already know that it was hands and fingers, you could not identify it, since the peripheral or side vision would not be capable of telling you. The nerve ends around the very outer circumference of the retina cannot give definite vision. If you move your hands five, inches forward, the wiggling fingers can now be seen, even with the gaze still held on the, door knob. You can recognise hands and fingers. Move the hands another five inches forward. In this position, you can count fingers and recognise jewellery because the hands are approaching the centre of sight. However, if your vision is fairly normal, you still see the door knob more definitely and more vividly than the fingers you can count on each side of your face. This proves the importance of the eyes keeping in true focus so that the centre of sight of each eye may be on the same object at the same time. If the eyes slip off focus and the outer retinal nerves take over, the image is blurred and indistinct.