THE EYE. 



is filled up with adipose or areolar tissue^ tlie eye is^ as already observed, 

 rarely quite spherical. The sclerotic may even be bouy in some well-ossiiied 

 forms of fishes, the bony portion usually consisting of two hollow half 

 spheres, having a hole posteriorly in the centre, for the entrance of the 

 vessels and nerves proceeding to the eye, while its anterior or outer emargi- 

 nation supports the cornea. In the swordfish XipMas, and in the Dipnoid 

 Ceratodus, these two bony half sphere's are confluent into one, having the 

 central foramen patent for the entrance of the vessels and nerves. 



The cornea, which is as a rule nearly ^flat, is a modified portion of the 

 corium, in some forms of fishes, at least in their earliest stage, as the shanny 

 Blennius pholis, at 1^- inches in length, the protrusion of the cornea equals 

 one-fifth of its diameter, and in a little rock goby {Gobius paganelhis) at 

 0"9 inches in length, the convexity to the diameter is as one to four. The 

 choroid tunics are three — the outer, or membrana argeutea, said to be 

 occasionally absent, and which is composed of acicular crystals, reflecting a 

 brilliant lustre, and often giving great brilliancy to the outer edge of the 

 iris ; the middle tunic, or membrana vasculosa, and also termed Haller's 

 membrane, is ramified with blood-vessels, while it supports the ciliary 

 nerve ; the inner tunic, or membrana picta, also termed uvea, is made up of 

 hexagonal and usually dark-coloured pigment cells. The ciliary processes 

 are rarely developed, but the ciliary ligament is always present. 



The iris is formed by a prolongation of the choroid membranes, having a 

 ciliary ligament at its base which overlies the convex border of the sclerotic. 

 It is a thin, contractile curtain,. situated behind the cornea, and suspended 

 in the aqueous humour. Its muscular structure is but feebly developed in 

 most fishes, consequently its capacity for dilating and contracting is not very 

 great. Its centre is perforated by the pupil for transmission of light to the 

 interior of the eye. This pupil is mostly large and circular, but in some forms 

 is vertically or horizontally elUptical, as in the majority of sharks, or it 

 may even be quadrangular, as in Oaleus. Occasionally a veil or flap descends 

 . from the edge of the iris at the upper portion of the pupil, as may be seen 

 in some pleuronectoids as the turbot, the viper weever fish of our coast, 

 or the more tropical crocodile fish, Platycephalus , or in a few rays as 

 the Homelyn (vol. ii, page 324). All these forms live more or less in the 

 sand, or at the bottom of the sea, and to them a large amount of light is 

 evideritly unsuited, consequently they are provided with an extensible and 

 retractile veil, which can regulate or entirely obstruct rays entering from 

 above. In the tropical Anableps the cornea, or window of the eye, is crossed 

 by an opaque horizontal band, and the iris appears to possess two pupils, a 

 subject I shall again have to advert to. On dissection, however, it is found 

 that- in reality the pupil has a lateral flap of the iris on either side, which, 

 although they cross each other) are not conjoined. 



