VISION. ■ • XXXI 



The retina is the delicate nerve structure upon which the image of 

 external objects is received. Its posterior or external stratum consists of a 

 cellular base supporting on its external or concave surface a layer of nerve 

 cells', the filaments from "which pierce the several superimposed granular 

 layers, the innermost of which consists, as stated by Owen, of interblended 

 twin-fusiform corpuscles . . . each of which is surrounded by a circle of 

 cylindricules. Over this latter the nerve fibres radiate without anastomosing, 

 and terminate in free ends at the base of the ciliary zone. A well-defined 

 raised' rim or bead runs along the anterior margin of the. retina, and also 

 the edge of the falciform slit. 



The aqueous humour in the anterior chamber is very small in amount, 

 owing to the constricted extent of the cavity occasioned by the projection 

 into it of the crystalline lens. But little fluid is necessary to float the free 

 border of the iris, while, due to the watery localities fishes inhabit, no 

 refractive aid is required to be placed anterior to the lens in the globe of the 

 eye. The vitreous body which fills up the greater portion of the cavity' of 

 the ball of the eye is of firmer consistence than observed in other vertebrate 

 animals. 



The choroid gland is a vascular ganglion of a deep red colour, resembling 

 in form a bent magnet ; it is placed between the outer and middle coats of 

 the choroid, and close to the optic nerve where it pierces those structures. 

 It is composed of parallel and .closely placed arteries and veins, the supply 

 of arterial blood for which is received from the pseudobranchias, and 'it 

 consequently is wanting in osseous fishes in which pseudobranchiaa are 

 absent, as sheat fish, carps, eels, &c. But in the chondropterygians and 

 ganoids there is no choroid, whereas pseudobranchias are present. 



The crystaline lens is nearly or quite spherical, very firm, having a dense 

 nucleus, and steadied in its position by the falciform process. It projects 

 into the anterior chamber of the eye which it almost fills, while it nearly 

 pressess against the flat cornea leaving but little room for any aqueous 

 humour. The refractive power of the lens is maximized by its spherical 

 form, to compensate for the deviation from the spherical form of the eyeball 

 produced by the flattening of its front portion. The fibres of the lens con- 

 verge at its two opposite ends, each having marginal teeth like those seen in 

 a cog-wheel, and by which they interlock with one another. In the Anableps 

 previously referred to, a slight bulging or excrescence of the lens occurs 

 opposite to what appears like the lower pupil of the eye. Inhabiting as fish 

 do a watery medium, the need of a lachrymal gland is scarcely obvious ; still an 

 instance of its existence in one of these creatures has been recorded. Situated 

 below the anterior corner of the eye is a. foramen, through, which is a 

 communication with the rudiments of one in the form of a blind sac placed 

 between the maxillary bone and the muscles of the cheek. 



