CHAPTER XXXII. 

 VISION. 



NEXT in importance to the intelligence we receive from the skin 

 is that which is conveyed to the brain from the outer world by 

 the second pair of cranial, or the optic nerves. 



The ending of the optic nerve differs from any of those we met 

 with in the skin, by being inclosed in a very specially arranged 

 organ the eyeball an apparatus for bending the rays of light, 

 so that they exactly reach the delicate sheet of complicated nerve- 

 ending which is here spread out. Nothing but the blood and 

 other tissues of the eye come in contact with the endings of the 

 optic nerve, which is thus placed out of the way of ordinary nerve 

 stimulation. Indeed, we shall see that the light, of which the 

 optic nerves convey intelligence to the brain, is not properly a 

 nerve stimulus, being merely the waving of an imponderable me- 

 dium, the existence of which is assumed. Besides the special 

 arrangements in the eyeball for bringing the rays of light to bear 

 on the nerve-endings, there must here be some extremely delicate 

 arrangement by which the ether-waves, that we call light, can be 

 converted into a nerve stimulus, or in some way made to affect 

 the nerve terminals in the retina. 



By means of the sense of sight we obtain knowledge of objects 

 at a distance from us, because all these objects reflect more or less 

 light, and thus make different impressions upon the terminals of 

 the optic nerve, which form the outer layer of the retina. 



Light, then, is the adequate stimulus for the retinal nerve- 

 endings, and the impulse caused by light is the only impression 

 the optic nerve is in the habit of carrying to our sensoria, where 

 the sensation of light is formed and distributed among the cells 

 of the brain so as to enable us to come to visual conclusions and 

 judgments. As already mentioned, no matter what stimulus, elec- 

 tric, mechanical, or other, be applied to the fibres of the optic 



