292 VITAL APPARATUS OF VISION. 



The vital apparatus of vision, situated in the 

 brain, is one of the largest parts of the latter 

 organ, being more considerable in size than the 

 hemispheres which are destined to the control of 

 the threefold faculty of Intellect, Sensation, and 

 Will. The nerve or conductor which passes 

 between the vital and the mechanical apparatus 

 of vision, is also of large size, and is curious in its 

 structure ; in external form it has the appearance 

 of a cylindrical rod, but when opened the cylinder 

 is found to be a mere sheath, containing a thin, 

 white membrane, plaited like a fan. When the 

 nerve reaches the eye-ball this membrane unfolds, 

 and is spread out upon the inner surface of the 

 back part of the eye, constituting the sentient 

 part of that organ. This white and spotless 

 membrane, upon which the rich picture of exter- 

 nal nature is received, and by which it is subse- 

 quently transferred for perception to the brain, is 

 the retina. 



Whether, therefore, we regard the mechanical 

 or the vital apparatus of the organ of vision, or 

 whether we pursue the inquiry by anatomical 

 investigation, or by observation of the habits of 

 the animals, we have the clearest evidence before 

 us, that the faculty of sight in fishes is one of their 

 highest sentient endowments. 



In examining the structure of the organ of 

 vision we are struck by the absence of the out- 



