CHAP, in.] SIGHT. 77 



I 



rise to visual sensations, and to visual sensations only. A me- 

 chanical stimulation of the retina, as when a blow is struck 

 on the eye, produces 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 ' 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 

 intensity, visual impulses, and thus visual sensations, are being 

 generated ; 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 apparently 

 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 which are 

 distributed to the optic nerve as nervi nervorum. Such ex- 

 periences are urged in support of the view that all impulses 

 passing along the optic nerve however generated, whether by 

 retinal changes or by other means, are visual impulses and visual 

 impulses only; they give rise to visual sensations and to visual 

 sensations alone. On the other hand, in other cases of removal 

 of the eye in the absence of anesthetics, neither section of the 

 optic nerve nor subsequent stimulation of the stump has given rise 

 to visual sensations. We shall return to this question later on 

 when we have to speak of what is known as the " specific energy 

 of nerves," and have only referred to it incidentally now. 



751. Visual sensations then may be produced in many 



