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 enclosed 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 are thus placed out of the way of ordinary nerve 

 stimulation. Further, 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 medium, the existence of 

 which is assumed. Besides the special arrangements in the eye- 

 ball 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 termi- 

 nals in the retina. 



By means of the sense of vision 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 con- 

 clusions and judgments. As already mentioned, no matter 

 what stimulus, electric, mechanical, or other, be applied to 

 the fibres of the optic nerve, the sensation produced is simply 



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