256 ELECTRO-PHYSIOLOGY 



persisted when the skin was stripped off before scalding to a 

 ligature at the foot, and then lifted back as far as the knee, after 

 dividing the cutaneous nerves, or removing them completely. 

 On the other hand, he succeeded (9) in demonstrating the negative 

 variation of the optic nerve on stimulating the retina with light, 

 in the eye of the pike, as, later on, in that of the perch, and still 

 more perfectly in the frog's eye. It is therefore certain that 

 the current of the sensory nerves reacts in this case to the 

 highly-specialised excitation of the epithelial end-apparatus by 

 light, exactly as that of the mixed or motor nerve reacts to 

 excitations of all kinds, it being immaterial whether the stimula- 

 tion proceeds from the central ganglion - cells, or whether it 

 impinges upon the continuity of the nerve itself, as a mechanical, 

 chemical, thermal, or electrical stimulus. In every case it is the 

 same negative variation of current in the nerve that appears 

 as the concomitant of excitation, and this is one of the main 

 supports of the prevailing theory of the physiological homo- 

 geneity of all nerve-fibres, and of the identity of the excitatory 

 process within them. It is a striking fact " that the optic nerve, 

 during the continuous stimulation of its terminal organ by light, 

 gives no different reaction from that of a tetanised nerve, under 

 discontinuous electrical stimulation. If in the last case we have 

 reason to believe the galvanometer inadequate to show the pre- 

 sumptive discontinuity of the variation, we must in the former 

 instance accept its evidence, since there is no reason to suppose 

 that the immediate effects of sustained illumination are, like most 

 other tetani, discontinuous " (Kiihne). 



This persistent diminution of current in the optic nerve 

 may be termed phototomis (Kiihne). It is a striking fact 

 that the close of the illumination, i.e. cessation of excitation 

 by light " or, more properly, the resumption of certain retinal 

 processes interrupted by light is also marked by a final negative 

 variation of the optic nerve, which cannot be interpreted other- 

 wise than as a repeated excitation traversing the nerve." If 

 " phototonus " is thus a sign of activity in the optic fibres, we 

 may justly conclude with Kiihne (l.c.) " that the cutting-off of light 

 is able to produce a greater effect upon the central organ, and to 

 discharge a more intense sensation (excitation), than the prolonged 

 action of the same light upon the eye." 



At the same time we cannot overlook the fact that the 



