FISHES. 333 



the superior edge of its pupil is prolonged into several 

 narrow stripes disposed in radii, gilded externally, and 

 black internally. In their ordinary state they are folded 

 between the superior edge of the pupil and the vitreous 

 humours ; but when we press the superior part of the eye 

 with the finger, they unfold themselves, and cover the pu- 

 pil like a window-blind. In the torpedo, the pupil can be 

 completely closed by means of this veil. No other fishes 

 possess any thing similar to this conformation, although in 

 most osseous fishes there is at each corner of the orbit a 

 vertical veil, which covers a small part of the eye. 



In general, the eyes of fishes are placed in a conical 

 cup, and repose on a mass of gelatinous matter contained 

 in a loose cellular substance. This trembling elastic mass 

 affords the eye a point of support in all its motions. In 

 the Chondropterygii) however, the eye is joined to the ex- 

 tremity of a cartilaginous stalk, which is itself articulated 

 in the bottom of the orbit. In this manner the muscles 

 act on a long lever, and have therefore great power in 

 moving the eye. 



The optic nerves arise under the cerebrum, and are 

 very large. They are composed either of distinct fila- 

 ments, or of a single flat band, which is sometimes folded 

 longitudinally on itself, and contracted into the figure of a 

 cord.' They cross each other without being confounded, 

 and we plainly see that the nerve of the left side proceeds 

 to the right eye, and that of the right side to the left. eye. 

 This crossing is less apparent in the cartilaginous fishes, 

 although in the ray the right nerve passes through an 

 opening in the left. These nerves pass directly through 

 the membranes of the eye by a round hole. Internally 

 they form a tubercle, which is papillated in the ray, sharks, 

 and carps. The radiating fibres, which arise from the 

 edges of these tubercles to form the retina, are very ob- 



