The photoreceptor process in vision.

In this paper I should like to review briefly the present status of the chemistry of visual excitation, specifically in vertebrates; and then to discuss connections between these processes and visual physiology and pa­ thology. Light initiates a nervous excitation in the outer segments of the rods and cones which, transmitted from one neurone to another to centers in the brain, ends in exciting visual sensations. To achieve this result probably the whole apparatus must be thrown into activity; yet all of it waits upon, and to a degree retains the impress of the primary processes of excitation in the receptor cells. The general arrangement of these proc­ esses is clear beforehand. Light, to have any effect, chemical or physical, must be ab­ sorbed. The rods and cones must contain substances which absorb visible light—hence pigments—and are changed thereby so as to yield a nervous excitation. The photosensi­ tive pigments must be continuously restored, or vision would cease soon after a light went on. The excitatory state must also be rapidly removed, or vision would continue long after a light went off. It would aid the economy of such a system if the latter reactions were coupled so as to complete a cycle, but this, though an advantage, is not an essential condition. All photoreceptor processes there­ fore are arranged as follows: