442 THE SENSORIAL FUNCTIONS. 



the posterior part, between its two portions, a con- 

 siderable 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 Sepiae 

 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 dif- 

 ference of density between the cornea and the 

 external medium is but small, the refractive power 

 of any cornea, however convex, would be incon- 

 siderable ; and the chief agent for performing 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 

 43Q nearly spherical, and of great den- 



sity. These circumstances are shown 

 in the section of the eye of the 

 Perch, Fig. 430.t The flatness of 

 the cornea leaves scarcely any 

 space for aqueous humour, and but 

 little for the motions of the iris. 



The surface of the eye in fishes, being con- 

 tinually washed by the water in which it is im- 



* See Cuvier, sur les Mollusques ; Memoir sur le Poulpe, p. 37. 

 In the Octopus there are folds of the skin, which appear to be rudi- 

 ments of eye-lids. 



t In this figure, as in the others, c is the cornea ; L, the lens ; v 

 the vitreous humour ; R, the retina ; o, the optic nerve ; and s, the 

 sclerotica. There is also found in the eyes of most fishes an organ, 

 lodged in the space k, termed the Choroid gland, which envelopes 

 the optic nerve, is shaped like a horse-shoe, is of a deep red colour, 

 and highly vascular ; its use is quite unknown. 



