PISCES FISHES. 



523 



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 sus- 

 pension of the iris : the vitreous humour, from 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 upon the lens. 

 B Fig. 231. C 



But the focus of the crystalline will be short in proportion as its 

 power is increased ; every arrangement has therefore been made 

 to approximate the retina to the posterior surface of the lens : 

 the eye-ball is flattened, by diminishing the relative quantity of 

 the vitreous humour ; and a section of the eye (Jig. 231 , B, c) 

 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 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 imbedded in its tissue, 

 which are sufficiently firm in their texture to prevent any alteration 

 in the shape of the eye-ball ; and in some of the large fishes the 

 sclerotic is actually converted into a cup of bone presenting orifices 

 at the opposed extremities, one for the insertion of the trans- 

 parent cornea, the other for the admission of the optic nerve. 



The vitreous humour and crystalline lens in many fishes are 

 kept in situ by a ligament placed for the purpose. This is a deli- 

 cate falciform membrane derived from the retina (Jig- 331, B, c), 

 which plunges into the vitreous humour, and, being continued along 



