VISUAL SENSATIONS. 765 



condition ; the photo-chemical effect may, as the photographers say, be 

 " fixed." 



It seemed very tempting, especially upon the first discovery of it, to sup- 

 pose that this visual purple is directly concerned in vision. If we suppose 

 that visual purple itself is inert toward the endings of the optic nerve, but 

 that either visual yellow or visual white, i. e., some product of the action of 

 light on visual purple, may act as a stimulus to those endings, the way seems 

 opened to understanding how rays of light can give rise to sensory impulses 

 in the optic nerve. Unfortunately visual purple is absent from the cones, 

 and from the fovea centralis, which, as we shall see, is the region of distinct 

 vision ; it is further entirely wanting in some animals which undoubtedly 

 see very well ; and, lastly, animals, such as frogs, naturally possessing the 

 pigment, continue to see very well, and even apparently to see colors, when 

 their visual purple has been absolutely bleached, as it may be by prolonged 

 exposure of the eyes to strong light. We cannot therefore, at present at 

 least, explain the origin of visual impulses by the help of visual purple. At 

 the same time its history suggests that some substances, sensitive like it to 

 light, but unlike it, colorless and therefore escaping observation, may exist, 

 and by photo-chemical changes be the means of exciting the optic nerves. 

 And, as we shall see later on, one theory of color vision is based on the 

 assumption that vision is carried on in some way or other by changes in 

 what may be called the visual substances present in the retina, these sub- 

 stances being used up and regenerated as vision is going on. 



But even admitting as probable the existence of these sensitive visual sub- 

 stances, the changes in which lead to stimulation of the real endings of the 

 retinal nervous mechanism, we cannot at present state anything definite con- 

 cerning those nerve-endings or the manner of their stimulation. It may be 

 that even the outer limbs of the rods and cones, in spite of the apparent 

 break of continuity between the outer and inner limbs, are really nervous 

 in nature. It may be, on the other hand, that the outer limbs are either 

 purely dioptric in function, or are associated with the sensitive visual sub- 

 stances in such a way that the purely nervous structures must be considered 

 as extending no further at least than the inner limbs. We cannot as yet 

 make any definite statement in the one direction or the other. 



655. In connection with the origin of visual impulses, we may per- 

 haps call attention to the remarkable changes which the cells of the retinal 

 pigment epithelium undergo under the influence of light. When an eye has 

 been shut off from all light for some little time the pigment is concentrated 

 in the bodies of the cells, and the remarkable filamentous processes of the 

 cells, with the pigment granules or crystals which they carry, extend a slight 

 distance only between the limbs of the rods and cones (about one-third down 

 the length of the outer limbs of the rods). Under the influence of light 

 these processes, loaded with pigment, thrust themselves a much longer way 

 down toward the external limiting membrane ; in consequence a considerable 

 quantity of pigment is found massed between the outer and even the inner 

 limbs of the rods and cones ; indeed, the outer limbs of the rods swelling at 

 the same time become jammed, as it were, between the masses of pigment, 

 causing the epithelial layer to adhere very closely to the layer of rods and 

 cones. 



656. The retina and optic nerve, like other nervous structures, develop 

 electric currents, which may be spoken of as currents of rest and currents 

 of action. They may be shown by placing one electrode on the retina of a 

 bisected eye, or on the cornea of a whole one, and the other on the optic 

 nerve, or hind part of the eyeball, or even on some distant part of the body. 

 They are also manifested by the isolated retina itself. The phenomena 



