76 SUPPLEMENT ON 



times irregular. The summit of the ball in rays is flat, form- 

 ing it into a kind of quarter of a sphere. That of anableps is 

 singularly formed ; having two corneas separated by an opaque 

 line, and two pupils opening in the same iris, so that it appears 

 double ; but there is but one vitreous humour, one crystalline, 

 and one retina. The crystalline of fishes is voluminous, leaving 

 little space for the vitreous ; the lens is very hard, and remains 

 transparent even in spirits of wine. The ball has four or five 

 tunics ; the sclerotic or external tunic is thick, fibrous, and in 

 most species supported by two cartilaginous pieces, wrapped 

 in its texture, which often becomes ossified. Anteriorly the 

 sclerotic opening surrounds the cornea ; the cornea is lamel- 

 lated as in other animals, the internal lamella being often 

 coloured yellow or green. Internally a thin membrane of a 

 golden or silver colour surrounds all the parts, and spreads 

 even before the iris, producing that beautiful metallic effect 

 generally visible in the eyes of fish. 



The pupil is but rarely provided with the faculty of altering 

 its diameter : rays and soles, however, have a singular pro- 

 duction, shaped like a palm upon the superior border of the 

 pupil, which can close the opening somewhat like a window 

 blind ; the posterior part of the iris consists of another mem- 

 brane, extending over the whole inside of the eye, and usually, 

 on the interior surface, coloured black. It can be divided into 

 two layers ; the internal, which is the true Ruyschian, and 

 the external, properly avascular and thicker substance, answer- 

 ing to the choroid. Between the last-mentioned membrane 

 and the metallic coat which envelopes it, there is an apparatus 

 solely found in osseous fishes, consisting of a band or roll vari- 

 ously twisted, and forming an irregular incomplete ring, encom- 

 passing the optic nerve, so as to represent a considerable 

 crescent, sometimes divided into two parts, but always open 

 at the inferior side. It is always very red, being composed 

 of a tissue of blood vessels. The use of this organ is un- 



