4.94 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 difter from those of sepias 

 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 Cuivier, sur les Mollusques ; Memoir sur le Poulpe, 

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

 to be rudiments of eye-lids. 



