ix ELECTRICAL EXCITATION OF NERVE 205 



With very strong currents Helmholtz found " a confusion 

 colours," in which no rule could be discovered. 



" The electrical excitation may also be confined to individual 

 parts of the retina, although it cannot be sharply localised. 

 The essentials of these manifestations have already been de- 

 scribed by Purkinje. Helmholtz made one conductor out of a 

 thin cylinder of sponge fully saturated with saline and tightly 

 bound to a copper rod with an insulating handle. The other 

 electrode was placed on the neck or grasped in the left hand, 

 while the sponge made contact with the skin near the 

 external or internal angle of the eye, which can be moved to 

 and fro under the closed eyelids. When the sponge is positive 

 electrode, the current passes on the proximal side of the eye 

 into and through the retina, leaving it again on the distal 

 side ; the reverse occurs when the sponge is negative. 



" The side of the retina at which current enters will then 

 appear more obscure than the half by which the current leaves 

 it, which is relatively brighter. It is to be remembered that 

 these sensations are always referred by the subject to the 

 opposite half of the field of vision, as if the electrical brightness 

 were due to external illumination. The same rules hold good 

 for the phenomena that occur when the electrode is placed 

 anteriorly upon the cornea, covered by the eyelid. The positive 

 electrode then gives current from within outwards, through the 

 entire retina, producing the sensation of brightness." Helmholtz 

 invariably found that the optic disc exhibited a contrary effect 

 from that of the surrounding field. 



"If positive electricity enters at the temporal side of the 

 eye, the current passes from without into the peripheral portion 

 of the retina, i.e. from cones to ganglion-cells, producing obscurity. 

 But in the fibres of the yellow spot that are directed towards 

 the temporal side, the current passes from ganglion-cells to cones, 

 and produces brightness. The several effects may be summed 

 up as follows : A constant electrical current through the retina 

 from cones to corresponding ganglion -cells gives a sensation of 

 darkness, the opposite direction of current a sensation of light" 



This, even more plainly than in the organ of taste, shows 

 the antagonism of sensations with opposite directions of current 

 in the same end-organ of the optic nerve. Any interpretation 

 other than dissimilar action at the two poles is hardly conceivable. 



