ORGAN OF SIGHT. 



115 





is an extremely delicate membrane (a), foUowed by a layer 

 of nerve-ceUs (&), from which the ter- 

 minal filaments issue, passing through 

 several granular strata (c, d, «), on which 

 the innermost stratum rests. This stra- 

 tum is composed of cylindrical rods (/) 

 vertically arranged, between which twin 

 fusiform corpuscles, (g) are intercalated. 

 This last layer is thickly covered with 

 a dark pigment. The retina extends 

 over a portion of the iris, and a well- 

 defined raised rim runs along its anterior 

 margin. 



The vitreous humour (Fig. 48, h) 

 which fills the posterior cavity of the eye- 

 ball, is of a firmer consistency than in the 

 higher Vertebrates. The lens is spheri- 

 cal, or nearly so ; firm, denser towards Fig. 49.— Vertical section 

 the centre, and lies in a hoUow of the °* ^'^^ '^^^% °^ *''« 



Beron, magn. A 350. 



vitreous humour. "When a falciform 

 process is present, it is with one end attached to the lens, 

 which is thus steadied in its position. It consists of con- 

 centric layers consisting of fibres, 

 which in the nucleus of the body 

 have marginal teeth, by which they 

 are interlocked together. In Petro- 

 myzon this serrature is absent, or 

 but faintly indicated. 



The anterior cavity of the eye 

 is very small in Fishes, in conse- 

 quence of the small degree of con- 

 vexity of the cornea ; the quantity 

 of the aqueous humour, therefore, is very smaU, just suffi- 

 cient to float the free border of the iris; and the lessened 



Fig. 50. — Interlocking fibres of 

 lens, higUy magnified. 



