10 THE SIGNS OF LIFE [LECT. 



they are both the tokens, I shall answer that in my opinion the 

 electrical indications are the more faithful ; they are without 

 doubt the more sensitive since they are still of quite consider- 

 able magnitude when a muscle is so weak as to scarcely con- 

 tract at all, nor be competent to raise a lever to any measurable 

 height. In the pair of records you have just seen, the electrical 

 indications are reduced to manageable size by shunting the 

 galvanometer. 



6. Retinal currents. The third object of experiment is a 

 frog's eyeball set up in a dark chamber between unpolarisable 

 electrodes which are connected with a galvanometer. By open- 

 ing a shutter the eyeball is illuminated at any desired time for 

 any desired period. When I do this, there is no mechanical 

 movement of the eyeball, at least no coarse mechanical move- 

 ment,* but you witness a large and obvious electrical change. 

 Holmgren, who first discovered this effect, found by a series of 

 experiments in which he made separate trial of the several 

 parts of the eyeball, that the electrical effect was of retinal 

 origin. Light acting upon the retina excites it ; in ourselves 

 the subjective token of that excitation is a sensation ; in the 

 isolated eyeball the objective token of excitation is an electri- 

 cal change. The detailed description of these changes will 

 form the subject of my next lecture, but I should be glad if 

 you would take note of what you have actually witnessed in 

 the present case. The eyeball has been set up on electrodes, 

 so that its fundus rests upon one electrode, while its cornea is 

 touched by the other electrode, the spot of light was deflected 

 to the left during the illumination, and returned to its original 

 position after illumination. I touch one terminal of the gal- 

 vanometer with a bit of zinc, and the other terminal with a 

 finger of the other hand ; the spot moves to the left, the ter- 

 minal I touched with the bit of zinc is that to which the 

 corneal electrode is connected : from which I conclude that 

 the current during illumination was from cornea to fundus. I 

 choose to call this direction " negative," and shall denote as 



* On closer examination, there may be detected (i) contraction of the 

 pupil, (2) protrusion of pigment, (3) retraction of cones. 



