CHAPTEE XII 



ELECTROMOTIVE ACTION IX THE EYE 



ALTHOUGH subjective observation and the analysis of sensations 

 must under all conditions take first rank among the methods of 

 investigating the physiology of the senses, the objective signs of 

 sensory activity, though scanty, are no less deserving of con- 

 sideration. It is true that the great expectations associated at 

 first with the discovery of visual purple, and the bleaching of it 

 by light, have not hitherto been fulfilled, and the later endeavours 

 of Konig and v. Kries to attribute a predominant role to this 

 " visual substance " must be regarded as discredited. 



Along with the " photo-chemistry " of the retina, its electro- 

 motive action and the alterations thereof by light stimuli claim 

 the chief share of attention. 



As early as 1849 du Bois-Eeymond investigated the electrical 

 reaction of the optic nerve, at its peripheral expansion in the 

 eye, while he was engaged in proving the identity of the 

 "natural" and artificial cross -section in nerve --as already 

 established for muscle. On leading off from the artificial section, 

 or from some point near it of the natural long section, of the 

 optic nerve, and the external surface {e.g. cornea) of a fish's eye- 

 ball as free from muscle as possible, the nerve was invariably 

 negative to the eyeball. The natural ending of the nerve thus 

 seems no more positive than the tendon end of the muscle-fibres. 



Sixteen years later, Holmgren (1) repeated these experiments, 

 using principally the frog's eye. He confirmed the observations 

 of du Bois-Eeymond in regard to the " current of rest," and 

 found, moreover, on leading off from the posterior portion of the 

 bulb and the optic nerve, that the former was weakly negative to 

 the latter. " If one electrode is applied to the optic nerve 



