884 FUSION OF VISUAL SENSATIONS. [BOOK m. 



images of which lie very close indeed to each other on the 

 retina. We distinguish the sensations, however, not by refer- 

 ence to the parts of the retina affected, but by reference to the 

 relations in space of the luminous points giving rise to the sen- 

 sations. Since this is a matter of some importance we may treat 

 of it in some detail. 



In the vast majority of cases the changes in the retina which 

 give rise to visual impulses, and so to visual sensations, are 

 brought about by light falling on the retina. But the retina 

 may be stimulated by other agencies than that of light. When 

 this is the case the changes in the retina, however produced, if 

 they are able to affect consciousness at all, give rise to visual 

 sensations, and to visual sensations only. A mechanical stimu- 

 lation of the retina, as when a blow is struck on the eye, pro- 

 duces a visual sensation, a sensation of light ; pressure exerted 

 on the eyeball so as to produce pressure on the retina gives rise 

 to visual sensations in the form of rings of light, of coloured 

 light, known as 4 phosphenes ' ; and when the retina is subjected 

 in various ways to stress or strain, as by rapid accommodation, or 

 by rapid movement of the eyeball from side to side, there often 

 result visual sensations in the form of light of some kind or 

 other, best appreciated when objective light is cut off from the 

 retina and when the retina has by long repose been rendered 

 unusually sensitive. Electrical stimulation also gives rise to 

 visual sensations ; not only is the induced current, or the break 

 and make of a constant current, thus able to excite the retina, 

 but during the whole time of the passage of a constant current 

 of adequate strength, even though it remain of uniform inten- 

 sity, visual impulses, and thus visual sensations, are being gener- 

 ated ; . in this respect the retina resembles sensory and differs 

 from motor nerves. It is stated that when the current is 

 directed from the layer of optic fibres to the layer of rods and 

 cones, the sensation is a positive one, a sensation of light or of 

 increased light, but that a current in the reverse direction gives 

 rise to a negative sensation, a sensation of diminished light, a 

 sensation of blackness. 



That the stimulation of retinal structures by other agents 

 than light may thus give rise to visual sensations, and appar- 

 ently to visual sensations alone, may be verified by experiment 

 at any time. The occasions on the other hand are rare in which 

 evidence can be gained as to whether stimulation of the optic 

 nerve apart from the retina, whether stimulation of the optic 

 fibres themselves, and not of their special endings in the retina, 

 also gives rise to visual sensations and to visual sensations alone. 

 In certain cases of removal of the eye it has been stated that 

 when the optic nerve was divided in the absence of anesthetics, 

 the patient " saw a great light " accompanied by no more pain 

 than could be accounted for by the filaments of the fifth nerve 



