CHAP, xii ELECTROMOTIVE ACTION IX THE EYE 171 



while the other is moved, touching now the back of the bulb 

 and now the cornea, there results in the first case what Holmgren 

 calls the ' weak reaction,' where the optic nerve is positive to 

 the eyeball, in the second the ' strong reaction,' nerve negative 

 to cornea." Holmgren rightly observes that retina can no more 

 than muscle be termed the " natural cross-section " of the cor- 

 related nerve, since it forms both anatomically and physiologically 

 an end-organ distinct from the nerve, capable conceivably of 

 independent electromotive action, like muscle or glands, under 

 certain conditions. If, therefore, in agreement with du Bois- 

 Eeymond's view, we are still to speak of natural transverse and 

 longitudinal sections, the former must be defined as the entire, 

 external, mosaic surface of the retina, bordering on the choroid, 

 while the inner boundary (layer of optic fibres) facing the 

 vitreous body forms the natural long section. Holmgren 

 determined the distribution of potential upon the surface of 

 the bulb with great accuracy, and endeavoured to bring the 

 retinal current into line with du Bois-Eeymond's law of the 

 muscle current. Without entering into detail, it may be said 

 that, as we should anticipate from the electromotive action of 

 the entire eye, there are in the retina differences of potential in 

 the direction of an " ingoing " current (i.e. from without, inwards), 

 signifying, in du Bois-Eeymond's sense, that the natural cross- 

 section (rods and cones) is negative to the natural long section 

 (internal surface of the retina). 



The inadmissibility of such an interpretation is, however, 

 obvious if we consider the structure of the retina, which, as 

 justly remarked by Kiihne and Steiner, " exhibits throughout, 

 and exclusively in the most external layer, objects that are quite 

 unlike free nerve -endings, while in no other layer is there 

 anything resembling such endings." 



Kiihne and Steiner (3) principally employed the isolated 

 retina of the frog in their successful researches. This tissue 

 may be slipped out of the fundus as out of a shell, with or 

 without pigment epithelium, its vital properties being preserved 

 intact, after which it can be drawn over a rounded glass rod, 

 and brought into contact with the leading-off electrodes. If 

 the rod -surface is external, and different points of the same 

 are tested, there is invariably a strong current between the 

 entrance of the optic nerve and the periphery, the former being 



