The question now arises, How do the retinae, or rather their outer layers, convert light into a nerve stimulus? It would appear quite out of the question that the 394 to 760 billions of waves of light per second could mechanically excite the nerve terminals as the waves of sound are believed to excite the endings of the auditory nerve. We know that light has a very distinct action on many chemical combinations, such as reducing salts of silver and gold, etc. We therefore imagine that the light waves may set up, in the outer layer of the retina, certain intermolecular motions or chemical changes, the result of which is that the nerve fibres are stimulated to activity and transmit an impulse to the brain. The light possibly produces a change in the outer layer of the retina which in some respects may be compared to that which occurs on a sensitive photographic plate. In some respects only, because, while the chemical change on the sensitive plate persists so as to give rise to a permanent photograph, in the eye it only lasts for the brief moment during which we can recognize the positive after image. The chemical change in muscle may be compared to the explosion of gunpowder, in giving rise to force, but not in the result produced in the materials. For in muscle the chemical change causing the contraction is rapidly repaired, while in the powder permanent alteration of the substance is produced. In the retina a new sensitive plate is at once produced by the restoration of the normal condition of the molecules, and similarly its explosive qualities are at once restored to the muscle.
The view that the layer of rods and cones undergoes a chemical change on exposure to light which suffices to excite the optic nerve, receives support from the observation that a color of a red or purplish hue exists in the outer part of the rods, and that this color changes when exposed to the light. But this so-called visual purple has not an inseparable connection with vision, since it is absent when the retina is most sensitive, i. e., the fovea centralis, where there are no rods, and further, frogs with blanched eyes seem to see quite well. Certain rays of light have a distinct thermic influence, and hence the possibility exists that the nerve impulse is started in the retina by some delicate thermic stimulus.
Against the chemical and thermic origin of the retinal stimulation may be urged the fact that the rays of the spectrum which are most efficient in exciting chemical and thermic variations (ultra violet and ultra red respectively) do not excite any nerve impulse in the retina.
The pigmented epithelial cells of the retina have been observed to change their shape slightly, and definitely to alter the position of the pigment granules they contain when exposed to light. When we remember how sensitive to light the protoplasm of many unicellular infusoria is, we cannot be surprised that the protoplasm of the retinal epithelium is affected by it. In the pigment cells of the frog's skin we are familiar with a change in shape and in the arrangement of their pigment granules in response to different light stimuli. We know further that in the nervous centres nerve impulses often originate in protoplasm under the influence of slight changes in temperature or nutrition. It would hardly be too much to assume, then, that the retinal epithelium has some important share in the transformation of light into a nerve stimulus. The arguments pointing to the rods and cones as the essential part of the retina apply equally well to the pigmented epithelium, for they are so dove-tailed one into the other that practically they form but one layer. They are not known to be connected with the nerve fibres, but they may still be influenced by the light, and communicate the effect to the contiguous nerve terminals, which appear to be elaborately adapted to the appreciation of subtle forms of stimulation.
Fig. 234. Epithelial cells of the retina, a, Seen from the outer surface; b, seen from the side, as in a section of the retina; c, shows some rods projecting into the pigmented protoplasm.