VITAL APPARATUS OF YISION. 223 



niche is absent, the eye is jointed to a moveable 

 bony stalk, and moves on the extremity of that 

 stalk as upon a ball and socket hinge. 



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 be- 

 tween 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 external nature is 

 received, and by which it is subsequently trans- 

 ferred 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 enquiry 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. 



