RESPONSE OF RETINA TO STIMULUS OF LIGHT 421 



thus becomes galvanometrically negative. The so-called 

 positive variation observed under such an experimental 

 arrangement, then, is really the result of the true excitation 

 of the retina conducted to the optic nerve. 



In experiments on nerve-and-retina preparations, with 

 contacts on the nerve and the retina, the existing injury- 

 current is from retina to nerve. When both nerve and retina 

 are excited simultaneously, by equi-alternating electrical 

 shocks, we have seen that, on account of the greater ex- 

 citability of the retina, the resultant responsive current, here 

 differential, is from retina to nerve, constituting a positive 

 variation of the existing injury-current (p. 419). In the case 

 of stimulation by light, however, it is the retina which is directly 

 excited. The added negativity thus induced in the retina 

 gives rise to an increase or positive variation of the existing 

 injury-current (fig. 250). Hence, in all these cases, the so- 

 called positive variation really indicates the normal excitatory 

 negative effect. The apparent anomaly involved in the 

 supposition that the response of the retina to light was 

 positive, and thus of opposite character to that of other ex- 

 citable tissues, is thus seen to be due to a misinterpretation 

 of observed results. It is unfortunate that, as a consequence 

 of this misinterpretation, the effects described by different 

 observers, of the response of the eye as * positive/ are really 

 to be understood as * negative/ and vice versa. When 

 quoting these results, therefore, I shall always give the actual 

 effect indicated in italics and in parentheses. 



Another observation which lends independent support 

 to the view that the retina exhibits the true excitatory re- 

 action, is found in the fact noted by Van Genderen-Stort 

 and Engelmann that the cones exhibit a motile effect by 

 retraction under light. It has been supposed from this that 

 the 'optic nerve contains not merely sensory, but also retino- 

 motor fibres. But I shall show that it is not that the endings 

 only of the optic nerve, but also that nerve itself, which 

 exhibit true excitatory contraction under stimulation (cf. 

 figs. 324, 404). All nerves, in fact, will be shown in a later 



