182 THE RIVER-SIDE NATURALIST. 



eye of a fish is the size of the crystalline lens and its 

 spherical form. " This shape," says Rymer Jones, " and 

 the extreme density of texture which the lens exhibits, are 

 indeed perfectly indispensable. The aqueous humour, being 

 nearly of the same density as the external element, would 

 have no power in deflecting the rays of light towards a 

 focus, and consequently the aqueous fluid in fishes is 

 barely sufficient in quantity to allow the free suspension of 

 the iris ; the vitreous humour, for the same reason, would 

 be scarcely more efficient than the aqueous in changing 

 the course of rays entering the eye, and hence the necessity 

 for that extraordinary magnifying power conferred on the 

 lens." But the focus of the crystalline will be short in 

 proportion as its power is increased ; every arrangement, 

 therefore, has been made to approximate the retina to the 

 posterior surface of the lens : the eyeball is flattened by 

 diminishing the relative quantity of the vitreous humour, 

 and a section of the eye shows that its shape is very far 

 from that of a perfect sphere. This flattened form could 

 not, however, have been maintained in fishes, had not a 

 special provision been made for the purpose in the con- 

 struction of the sclerotic. The outer tunic of the eye, 

 therefore, generally contains two cartilaginous plates em- 

 bedded in its tissue, which are sufficiently firm in their 

 texture to prevent any alteration in the shape of the eye- 

 ball. The vitreous humour and crystalline lens in many 

 fishes are kept in situ by a ligament placed for the purpose. 

 In some fishes, as the salmon, this ligament is of a dark 

 colour. 



Another peculiarity in the eye of the bony fishes is the 

 presence of a vascular organ at the back of the eyeball, 

 interposed between the choroid tunic and a brilliant metallic- 

 coloured membrane, which invests the choroid externally. 

 This organ is of a crescent form, and always of a deep-red 

 colour, principally made up of bloodvessels, always much 

 ramified, forming a vascular network in the choroid. The 

 nature of this organ is uncertain ; some suppose it to be 

 muscular, others glandular. Professor Rymer Jones thinks 

 it an erectile tissue analogous to that of the corpus caver- 



