SEC. 2. VISUAL SENSATIONS. 



Light falling on the retina excites sensory impulses, and these 

 passing up the optic nerve to certain parts of the brain, produce 

 changes in certain cerebral structures, and thus give rise to what 

 we call a sensation. In a sensation we ought to be able to dis- 

 tinguish between the events through which the impact of the rays 

 of light on the retina is enabled to generate sensory impulses, and 

 the events, or rather series of events, through which these sensory 

 impulses (for, judging by the analogy of motor nerves, we have no 

 reason to think that they undergo any fundamental changes in 

 passing along the optic nerve), by the agency of the cerebral 

 arrangements, develope into a sensation. Such an analysis, how- 

 ever, is, at present at least, in most particulars, quite beyond our 

 power ; and we must therefore treat of the sensations as a whole, 

 distinguishing between the peripheral and central phenomena, on 

 the rare occasions when we are able to do so. 



The Origin of Visual Impulses. 



Of primary importance to the understanding of the way in 

 which luminous undulations give rise to those nervous changes 

 which pass along the optic nerve as visual impulses, is the fact that 

 the rays of light produce their effect by acting not on the optic 

 nerve itself but on its terminal organs (see p. 488). They pass 

 through the anterior layers of the retina apparently without induc- 

 ing any effect ; it is not till they have reached the region of the 

 rods and cones that they set up the changes concerned in the 



