SIGHT. 317 



The Visual Apparatus consists of nervous tissues immedi- 

 ately concerned in "giving rise to sensations, supported, 

 protected, and nourished by other parts. Its essential parts 

 are, (1) the retina, a thin membrane lying in the eyeball and 

 containing microscopic elements which are so acted upon 

 by light as to stimulate (2) the optic nerve ; this nerve ends 

 (3) in a part of the brain (visual centre) which when stimu- 

 lated arouses in our consciousness a feeling or sensation of 

 sight. The visual centre may be excited in very many ways, 

 and quite independently of the optic nerve or the retina ; 

 as is frequently seen in delirious persons, in whom inflamma- 

 tion or congestion of the brain excites directly the visual 

 centre and gives rise to visual hallucinations. 



Usually, however, the cerebral visual centre is only ex- 

 cited through the optic nerve, and the optic nerve only by 

 light acting upon the retina. The eyeball, containing the 

 retina, is so constructed that light can enter it, and so placed 

 and protected in the body that as a general thing no other 

 form of energy can act upon it so as to stimulate the retina. 

 Under exceptional circumstances we may have sight-sensa- 

 tions when no light reaches the eye; anything which stimu- 

 lates the retina, so long as it is connected by the optic nerve 

 with the cerebral visual centre, will cause a sight-sensation. 

 A severe blow on the eye, even in complete darkness, will 

 cause the sensation of a flash of light; the compression of the 

 eyeball excites the retina, the retina excites the optic nerve, 



Of what does the visual apparatus consist? What are its essen- 

 tial parts? What happens when the visual centre is stimulated? 

 Is it only stimulated by the agency of light? Illustrate. 



How is the visual centre usually excited? Why is light the form 

 of energy which most often stimulates the retina? Give an example 

 of the production of a sight-sensation in the absence of light. What 

 happens in the nervous system when a man "sees sparks" on receiv- 

 ing a blow in the eye? 



