POLAR EXCITATION OF SENSE ORGANS. 517 



effect is appreciated when non-polarisable electrodes are used with a 

 current intensity of only T J^ milliampere. 1 



The effects may be due to the excitation of specific taste organs, or to the 

 presence of electrolytic products. It is undoubtedly due to the stimulation of 

 the peripheral organ at the pole, and not to excitation of the larger branches 

 of the gustatory nerves, since it is much diminished by the local application 

 to the surface of cocain, which dulls all taste sensations. That it is not the 

 mere effect of acid and alkali, due to electrolytic decomposition, is shown by 

 the circumstance that the sensations are present when no evidence of such 

 chemical products can be found. 2 



The change in the sensation on the cessation of the current may be caused 

 by the reversed polarisation current coming into play, but in any case it is 

 of the nature of an opening excitatory change. The production of greater 

 instability (dissimilation process) in a peripheral sense organ constitutes an 

 excitation of a specific kind, and the resultant excitatory state, when propagated 

 to the higher centres, is thus the physiological basis of a specific sensation. 

 If the production of greater stability (assimilation process) is also an excitation, 

 it must be of fundamentally different character. Both the propagated nerve 

 states and the result in consciousness will thus differ from those associated 

 with greater instability. The two states are linked together, since the 

 production of either is followed by a rebound into the other condition. This 

 is the view held by Hering in reference to visual contrast, and the taste 

 effects are so nearly parallel, as far as the antagonism of effect with after-effect 

 is concerned, as to fall in with his conceptions. 



The visual organs. If the poles of a galvanic current of sufficient 

 intensity are placed one on the forehead and one on the neck, both the 

 closure and the opening cause an effect which appears to consciousness 

 as the sensation produced by a flash of light. More intense currents 

 produce effects not only at but during closure. These differ with the 

 direction of the current, and are best obtained when one electrode is 

 placed over the shut eyelid. 



There is considerable disagreement as to the character of the sensa- 

 tion experienced under these conditions, but the majority of observers 

 concur in the following particulars. 3 When the anode is on the eyelid, 

 the flash at closure has the appearance of a violet field with dark centre. 

 This is succeeded by field illumination during the maintenance of the 

 current flow, which slowly subsides, and often appears of a violet or 

 blue hue. On opening the current, there is a bright blue centre with 

 an outside field of yellowish green, or, according to Helmholtz, orange. 



With the cathode on the eyelid, the effects are more or less reversed. 

 At the instant of closure the centre is bright and blue, the outside field 

 dark. This persists, but with diminishing intensity, during closure, 

 and, on cessation of the current, is succeeded by opposite effects the 

 centre being greenish yellow and the external field blue. Hence the 

 centre appears of a hue in contrast with its surrounding, and is, 

 according to Helmholtz, associated as to its peripheral origin with the 

 entry of the optic nerve or blind spot. On this supposition it 

 represents a retinal area which is incapable of appropriate stimula- 

 tion by the current. 



The results as to the visual characters of the field appear to show 



1 Laserstein, Arch. f. d. ges. Physiol., Bonn, Bd. xlix. S. 519. 



2 v. Vintschgau, see Hermann, ' ' Handbuch, " Bd. iii. (2), S. 186. 



3 Helmholtz, " Physiol. Optik," 2te Aufl. S. 243 et seq. 



