VISION INTRODUCTION 



617 



demonstrated in Wald's chapter. The pro£!;ress that 

 has been made in the study of chemistry of the primary 

 photosensitive substances of the rods and cones of the 

 vertebrate retina, and the receptors of a few inverte- 

 brates, is indeed impressive. 



It was a significant step when the visual pigments 

 could be extracted from their loci in the outer limbs 

 of the rods and cones, and bleached and resynthe- 

 sized in vitro. Another important step has now been 

 taken by Rushton and his colleagues (14), who have 

 succeeded in measuring the bleaching and regenera- 

 tion of visual pigments of both rods and cones in the 

 living eye, as described in Wald's chapter. Operating 

 on the principle of the ophthalmoscope, a sensitive 

 photoelectric device is used to measure the light re- 

 flected back through the retina of a human subject. 

 Rushton's studies are providing a link between the 

 biochemical knowledge of the visual pigments, and 

 the physiology of the living retinal receptors. 



Biochemistry alone is not sufficient to solve the 

 problem of the photoreceptor. In the living eye, 

 visual pigments are part of highly organized cellular 

 systems. New concepts of the fine structure of visual 

 receptor cells are emerging from recent cytological 

 investigations. In the developing vertebrate retina, 

 the rods and cones originate as ciliated epithelial 

 cells from the neural tube. [This subject has been 

 reviewed by Detwiler (2) and by Walls (17).] The 

 cilia becoine transformed (i) into the outer segments, 

 which are long stacks of doui^le-membrane disks 

 (15). Remnants of the original ciliary structure re- 

 main visible to electron microscopy in the com- 

 pletely developed receptors (i, 12). In arthropods, the 

 osmium-staining ' membranes' take the form of 

 densely packed microvilli of the surfaces of the retinula 

 cells, so that the rhabdom has a structure resembling 

 a honey-comb (4, 10, 18). Rhodopsin is present only 

 in the outer segments of the rods, and, as Wald 

 points out in his chapter, constitutes a large fraction 

 of their bulk. Prcsumaijly a similar arrangement of 

 visual pigment holds for the cone outer segment 

 and for the invertebrate rhabdomere as well. These 

 cytological facts will have to be taken into considera- 

 tion in any theory of the receptor mechanism. 



.•\ photoreceptor is a transducer of light energy 

 into nervous action. The first step, the photochemical 

 change in a specific visual pigment, is now quite 

 familiar. The later steps, ultimately resulting in 

 nervous excitation that is transmitted in the afferent 

 nerve fibers, are almost completely unknown. Wald 

 and Granit in their chapters have indicated some of 



the possibilities that are to be considered [see also 

 (5) and (7)]. Presumably at least some of these proc- 

 esses in the photoreceptor are not basically different 

 from those occurring in any other cell of the nervous 

 svstem. Indeed, it would not be surprising if the 

 entire photosensitive mechanism were the result of 

 but a comparatively minor modification of a funda- 

 mental irritable structure of a cell. The photosensi- 

 tivitv of some ganglion cells, as discussed in the 

 Milnes' chapter, and the fact that peripheral nerves 

 can be photosensitized by dyes (3) makes this a not 

 unreasonable expectation. 



The final outcome of the excitatory processes ini- 

 tiated by light is the generation of trains of nerve 

 impulses in the fibers of the optic pathway. Whether 

 all photoreceptor cells themselves — the rods and 

 cones in the vertebrate retina, the retinula cells in 

 the arthropod compound eye, for example — actually 

 generate trains of discrete impulses in their own fibers 

 is not established; but some primary receptor cells do, 

 and so do neurons closely associated with the recep- 

 tors. Optic nerve fiber activity consists of the rhyth- 

 mic succession of propagated ' all-or-none' disturb- 

 ances typical of the activity of all neurons concerned 

 with transmitting influences rapidly over large dis- 

 tances. Studies of the discharge of impulses in single 

 optic nerve fibers have shown that many of the 

 familiar phenomena of vision have their origin in 

 properties of the receptors, or of the retinal neu- 

 rons (6). 



Intimately associated with the excitation of the 

 visual mechanism are comparatively slow electrical 

 changes measurable grossly as the retinal action po- 

 tentials. These are discussed in Granit's chapter. As 

 a result of studies employing microelectrodes that 

 are small enough in some instances to penetrate 

 single cells and record electrical activity from within 

 them, the .significance of various components of the 

 retinal action potentials is gradually becoming clearer. 

 It seems likely that an integral link in the excitatory 

 process is a change in electrical polarization of cellu- 

 lar structures, brought aijout somehow by the photo- 

 chemical system of the receptor. As in other parts of 

 the nervous system, these electrical changes, because 

 of the local current flow they engender, result in the 

 initiation of relaxation oscillations in cellular mem- 

 branes which, conducted, are the trains of nerve 

 impulses that constitute the sensory message to the 

 higher centers. 



An eye is more than a simple mosaic of photore- 

 ceptor elements. The histological complexity of the 



