VISION or SIGHT, the function, in physiology, of the organ known as the eye (q.v.). The sense of vision is excited by the influence of light on the retina, the special terminal organ con nected with the optic nerve. By excitation of the retina, a change is induced in the optic nerve fibres, and is conveyed by these to the brain, the result being a luminous perception, or what we call a sensation of light or colour. If light were to act uniformly over the retina, there would be no image of the source of the light formed on that structure, and consequently there would be only a general consciousness of light, without reference to any particular object. One of the first conditions, therefore, of vision for useful purposes is the formation of an image on the retina. To effect this, just as in a photographic camera, refrac tive structures must be placed in front of the retina which will so bend luminous rays as to bring them to a focus on the retina, and thus produce an image. Throughout the animal kingdom various arrangements are found for this purpose; but they may be all referred to three types, namely—(i) eye-specks or eye dots, met with in Medusae, Annelidae, etc.; (2) the compound eye, as found in insects and crustaceans; and (3) the simple eye, common to all vertebrates. The eye-specks may be regarded simply as expansions of optic nerve filaments, covered by a trans parent membrane, but having no refractive media, so that the creature would have the consciousness of light only, or a simple luminous impression, by which it might distinguish light from darkness. The compound eye consists essentially of a series of
transparent cone-like bodies, arranged in a radiate manner against the inner surface of the cornea, with which their bases are united, while their apices are connected with the ends of the optic fila ments. As each cone is separated from its neighbours, it admits only a ray of light parallel with its axis, and its apex represents only a portion of the image, which must be made up, like a mosaic-work, of as many parts as there are cones in the eye. The size of the visual field will depend on the size of the segment of the sphere forming its surface.
The eyes of many insects have a field of about half a sphere, so that the creature will see objects before and behind it as well as those at the side. On the other hand, in many the eyes have scarcely any convexity, so that they must have a narrow field of vision.
For numerous anatomical details, and various diseases connected with the eye, see EYE; the pathological aspects of vision itself are treated at the conclusion of this article.