STRUCTURE OF THE EYE OP FISHES. 



657 



met with in the human eye, and, generally speaking, arranged in the 

 same manner as in Man. It is not our intention, therefore, in the fol- 

 lowing pages minutely to describe the anatomy of the eye in every class 

 which will come under our notice ; but taking the human eye, with the 

 construction of which we presume our readers to be intimately acquainted, 

 as a standard of comparison, point out those modifications of the general 

 type of structure common to this division of animated nature. 



(1808.) The first thing which strikes the attention of the anatomist, 

 when examining the eye of a fish, is the size of the crystalline lens, and 



Fig. 321. 



Structure of the eye in Fishes. 



its spherical form. This shape, 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 ele- 

 ment, 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, 

 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. 



(1809.) 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 eyeball 

 is flattened, by diminishing the relative quantity of the vitreous humour ; 

 and a section of the eye (fig. 321, 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 construction of the sclerotic : the outer tunic of the 



2u 



