THE FISHES. 181 



The eyes of all vertebrate animals are constructed 

 upon principles essentially similar, and have, as a rule, 

 the same parts as are found in the human eye, and 

 only modified according to circumstances. The following 

 short description of the eye in man will assist in under- 

 standing that of the fish. The globe or, as it is more 

 generally termed, the ball of the eye is composed of 

 membranes placed one within the other, and of humours 

 or fluids which they enclose. The membranes are the 

 conjunctiva which lines the free border and inner surface 

 of the eyelids. The sclerotic, a fibrous membrane, firm 

 and resistant in texture, extending from the entrance of 

 the optic nerve to the border of cornea, which occupies the 

 anterior fifth of the globe of the eye. The anterior sur- 

 face is convex and prominent, is in contact with the con- 

 junctiva; the posterior is concave, and is lined by the 

 membrane of the aqueous humour. The choroid mem- 

 brane, vascular and composed of minute arteries and veins, 

 united by cellular tissue, lies between the sclerotic and 

 the retina, which is placed between the choroid membrane 

 and the vitreous humour soft and pulpy in its structure, 

 and transparent in the living subject. The iris, or coloured 

 circle, seen through the transparent cornea, resembles a 

 partition to divide the interval between the cornea and 

 lens into two parts ; the interval is filled with the aqueous 

 humour, and the iris moves freely in it. The space 

 between it and the cornea is called the anterior chamber, 

 which is the larger ; that behind, the posterior chamber. 

 The aqueous humour is a thin pellucid fluid, filling up 

 the two chambers of the eye, occupying the space between 

 the cornea and crystalline lens, which is placed at the union 

 of the anterior third with the two posterior thirds of the 

 eye behind the iris, and embedded in the vitreous humour, 

 which fills up the posterior two-thirds of the globe of the 

 eye. It consists of a thin transparent fluid, enclosed in a 

 membrane, called the hyaloid membrane. This structure, 

 although thin and transparent, is firm, particularly in the 

 forepart. 



That which strikes the attention when examining the 



