vi i RETINAL EXCITATION 349 



with Lviihuo as a heliotactic or phototactic phenomenon, because, 

 as we have seen, it may be induced reflexly, and not by light 

 alone, but by other physical and chemical stimuli. Chiarini's 

 hypothesis seems more acceptable. He regards the migration 

 of the pigment and the greater cohesion between the retinal 

 epithelium and the layer of rods and cones in animals influenced 

 by sunlight, as a chemotactic phenomenon, by which the rods and 

 cones are provided with the materials necessary to make good the 

 loss suffered during their functional activity. This view, supported 

 also by Pergens and others, agrees with the observations of Boll 

 and Kiihne that the retinal epithelium is indispensable to the 

 regeneration of the purple, although the latter is at least to 

 some extent independent of the fuscin, since it exists in albino 

 rabbits, in which the epithelium is altogether destitute of pigment. 



In addition to the nutritive function, Angelucci assigned to 

 the migration of the fusciu the office of protecting the sensitive 

 elements of the retina from unduly intense light, and of counter- 

 acting the effect of dazzle in the eyes of albinos which have no pig- 

 ment. But this protective function is common to the pigment 

 of the choroid and iris as well as to the fuscin, and must be 

 independent of its movement. And, on the other hand, we have 

 seen that the protective role of fuscin can only be rudimentary in 

 mammals and in man, where the retinal pigment is very scanty, 

 so that the rods and cones are almost entirely exposed (Fig. 154). 



The retinal epithelium undoubtedly plays a certain part in 

 the adaptation of the eye to intense light and twilight. " When 

 the retina is affected by strong light it functions with greater 

 activity ; its nutritive exchanges are more active and its consump- 

 tion greater; and to provide for this increased consumption the 

 connection between the retinal epithelium and the pigment on 

 the one hand, and the layer of rods and cones on the other, is 

 more intimate. When, on the contrary, it is exposed to weak 

 light, or better is kept in complete darkness, its functional 

 activity is diminished or suspended, its nutritive exchanges are 

 less active, its consumption is reduced to the lowest terms, and in 

 consequence the connection between the epithelium and the rods 

 and cones is less intimate " (Chiarini). That is, positive chemotaxts 

 in the first case, negative chemotaxis in the second. 



The electromotive jjhenomena that can be observed in the retina 

 are connected with the contraction of the retinal epithelium and 

 the contracted segments of the cones under the action of light. 



As early as 1849 du Bois-Eeymond observed that when an 

 entire living eye, or the isolated retina of any vertebrate, was 

 brought into circuit, by means of unpolarisable electrodes with a 

 good galvanometer, a current, which he termed current of rest, could 

 be led off. According to Holmgren (1866-71), on joining up the 

 cornea of the frog's eye and the stump of the optic nerve, the 



