Next in importance to those impulses which we receive from the skin are those conveyed to the brain from the outer world by the second pair of cranial, or the optic nerves.
The ending of the optic nerve differs from any of those met with in the skin, by being enclosed in a very specially arranged organ - the eyeball - an apparatus for bending the rays of light, so that they exactly reach the delicate sheet of complicated nerve ending which is here spread out. Only the blood and other tissues of the eye come in contact with the endings of the optic nerve, which are thus placed out of the way of ordinary nerve stimulation.
Further, the light, of which the optic nerves convey intelligence to the brain, is not properly a nerve stimulus, being merely the waving of an imponderable medium, the existence of which is assumed. Besides the special arrangements in the eyeball for bringing the rays of light to bear on the nerve endings, there must here be some extremely sensitive arrangement by which the ether waves, which we call light, can be converted into a nerve stimulus, or in some way made to affect the nerve terminals in the retina.
By means of the sense of sight we obtain knowledge of objects at a distance from us, because all these objects reflect more or less light, and thus make different impressions upon the terminals of the optic nerve forming the outer layer of the retina.
Light, then, is the adequate stimulus for the retinal nerve endings, and the impulse caused by light is the only impression the optic nerve is in the habit of carrying to our sensoria, where the sensation of light is formed and distributed among the cells of the brain, so as to enable us to come to visual judgments and conclusions. As already mentioned, no matter what stimulus electric, mechanical, or other, be applied to the fibres of the optic nerve, the sensation produced is simply light, and this is thought of as if it came through the eye from the outer world.
The study of the function of vision may be divided into:
1. The path the light takes on its way through the eye to reach the retina.
2. The molecular changes in the retina which give rise to stimulation of the optic nerves.
3. The sensations arising in the sensorium as the result of the molecular changes set up in the cerebral nerve cells by the impulses from the optic nerve.
4. The visual perceptions and judgments which our consciousness is capable of elaborating from the visual sensations.