494 THE SENSORIAL FUNCTIONS. 



distinct iris, with a kidney-shaped pupil. This 

 eye is remarkable for the total absence of a 

 cornea ; the integuments of the head being 

 continued over the iris, and reflected over the 

 edges of the pupil, giving a covering to the ex- 

 ternal surface of the lens ; there is, of course, 

 no chamber for containing an aqueous humour. 

 The globe of the eye is nearly spherical ; but the 

 sclerotica is double, leaving, at the posterior part, 

 between its two portions, a considerable space, 

 occupied by the large ganglion of the optic 

 nerve, with its numerous filaments, which are 

 embedded in a soft glandular substance.* 



The eyes of Fishes differ from those of Sepise 

 principally in the addition of a distinct cornea, 

 exterior to the lens and iris, but having only a 

 slight degree of convexity. This, indeed, is the 

 case with all aquatic animals ; for, since the 

 difference of density between the cornea and 

 the external medium is but small, the refractive 

 power of any cornea, however convex, would be 

 inconsiderable ; ,and the chief agent for per- 

 forming the requisite refraction of the rays is 

 the crystalline lens. We accordingly in general 

 find the cornea nearly flat, and the globe of the 

 eye approaching in shape to a hemisphere ; 

 while the lens itself is nearly spherical, and of 



* See Cuvier, sur les Mollusques ; Memoir sur le Poiilpe, 

 p, 37. In the Octopus there are folds of the skin, which appear 

 to be rudiments of eye-lids. 



