xii ELECTROMOTIVE ACTION IX THE EYE 479 



there was never an effect upon the galvanometer, since the 

 photo -electrical variation occurred during the period of ;m 

 entire revolution (0'25G4 sec.). It further appeared "that a 

 measurable period (0'0005-0'0060 sec.) elapsed between the 

 moment of stimulation and the perceptible commencement of 

 the positive part of the variation, after which the positive 

 preliminary phase rapidly attains its maximum, and then falls 

 quickly, passing into the negative portion of the photo-electrical 

 variation. If this latter makes its appearance alone, with no 

 positive fore-swing, a stage of latent excitation (from 0'0004 to 

 Q'0064 sec.) is still plainly visible, followed first by a weaker 

 phase (negative fore -swing) and then by the true negative 

 principal variation. The maximum duration of the positive 

 fore-swing lasts, according to Fuchs, O'OISI sec.; the minimum 

 = O'OOVO sec. The negative fore-swing lasts (with exclusive 

 appearance of the negative variation) between 0-0029 and 

 O'OIOS sec.; the period to the maximum of the negative varia- 

 tion = 0-0089-0-0352 sec. 



The question may be raised as to what portions (layers) 

 of the retina are mainly or solely concerned in initiating 

 the electrical P.D. The exclusive appearance of the negative 

 variation at the trunk of the optic nerve on stimulating the 

 eye with light, permits us to conclude with tolerable certainty 

 that the anterior fibrous layer gives a similar reaction, whence 

 it follows that the processes underlying the complex photo- 

 electrical variations of the retina are situated in layers that 

 do not extend anteriorly beyond the ganglion layer. That 

 this layer is not itself directly implicated appears from the fact 

 that retinal preparations from warm-blooded animals are usually 

 very unstable, and quickly lose their photo-electrical reaction 

 even when the whole fuudus of the eye is examined. This 

 must be referred to the well-known sensibility of non-ganglionic 

 elements to all disturbances of their normal metabolism. The 

 ///'//// resistance of the bird's retina is therefore the more 

 remarkable. Kiihue and Steiner obtained good results with 

 even the isolated retina (pigeon), which can only be referred to 

 the great vitality of the long rods and cones in the pigeon, 

 since it can scarcely be supposed that ganglion-cells or nerve- 

 fibres of the retina would be excitable 45-50 rnin. after making 

 the preparation. We are thus forced to conclude that the 



