THE EYE OF MAN. 385 



range of observation. Originating in the optic thalami, two 

 round eminences situated to the right and left at the basis of 

 the brain, they cross each other ; and then each of them, proceed- 

 ing to the eye for which it is destined, spreads out into a mem- 

 brane gifted with marvellous powers a truly magical mirror, 

 which, assisted by the most wonderful optical contrivances, ex- 

 hibits to the mind a faithful picture of the external world in all 

 its infinite varieties of form and colour. On this small surface a 

 vast prospect reflects itself with photographic distinctness, and 

 our mind receives the impression, and translates immediately the 

 little picture into the vast reality, and steps, as it were, out of 

 its own frame into the wide space submitted to its view. The 

 charms of a splendid landscape, the magnificence of the ocean, 

 the awful grandeur of the heavens, may well strike us with 

 admiration ; but if custom had not made us too familiar with 

 the inexplicable wonders of our own organisation, we should 

 be no less enraptured with the amazing instrument which 

 thus wonderfully reveals to us the beauties of the external 

 world. What a surprising delicacy of sensation \ how perfect 

 a structure ! and how admirable the strength of the healthy 

 eye, before whose crystal mirror picture follows upon picture 

 for hours together, without its ever being fatigued, or losing 

 anything of the sharpness of its perceptions ! 



From another part of the basis of the brain proceed two other 

 nerves, differently though 

 no less admirably gifted. 

 These penetrate on each 

 side into a cavity of the 

 skull, forming the hard 

 solid case of a most cu- 

 riously-constructed appa- 

 ratus, which every vibra- 

 tion of the air sets into 

 a corresponding motion. 

 Here, in a bony struc- 

 ture very similar to a 



, ,- . . Expansion of the Acoustic Nerve. (Magnified.) 



snails shell, the innu- 

 merable filaments of the acoustic nerves expand into a spiral 

 membrane, which, stimulated by the vibrations of the sounding 



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