Peculiar Structure in the Eyes of Fishes. 137 



from its peculiar connection with the lens, (more or less di- 

 rectly varying in different tribes), may fairly be presumed to 

 exert an immediate influence on the position of this refractive 

 body. 



In fish, however, the eye differs in many essential respects 

 from that organ in land animals. The cornea is nearly flat, 

 while the sclerotica is often cartilaginous, or so thick as to be 

 but little acted on by the feeble muscles inserted into it. The 

 iris possesses no appearance of fibre, (in the true fishes), and 

 its vessels are not only less numerous, but present no trace of 

 that remarkable arrangement, on which, in land animals, the 

 erectility of tissue has been presumed to depend. There are 

 no ciliary processes. The lens is nearly, if not altogether, 

 spherical. There is, indeed, a new, and doubtless an impor- 

 tant organ, superadded ; this is the choroid gland, as it has 

 been called : it is of considerable size, and great vascularity, 

 but its functions are, as yet, undetermined. 



In those fish which I have had an opportunity of observ- 

 ing in their living state, I could never detect any movement 

 of the iris, in accordance with the degrees of light ; although 

 I am by no means prepared to deny the possibility of such 

 action. Still, the absence of those organs, supposed by ma- 

 ny to exercise an influence in determining the focal adjust- 

 ment of the lens in land animals, viz. an active iris, ciliary 

 processes, and a marsupium, might lead us to enquire whe- 

 ther any other means exist, by which the organ might be a- 

 dapted to vision at different distances. The fact of the sphe- 

 ricity of the lens, would induce us to suspect that the power 

 of adjustment, if any, would be fine ; a very inconsiderable 

 movement in a lens so shaped, making a great difference in 

 the distinctness of the image refracted ; or, in other words, 

 making a sensible alteration in its focus. 



Some few years back, in examining the organ of vision in 

 a pike, (Esox lucius), I observed a small, roundish, grey-co- 

 loured body, about the size of a hemp-seed, attached to the 

 circumference of the lens ; and at that time, certainly without 

 due consideration, I designated it a muscle, principally from 

 the fact, that I traced a nerve running from the posterior part 

 of the eye, to this peculiar body. The preparations then 

 made, I exhibited to some young American gentlemen, at- 

 tending the practice of the Moorfields Ophthalmic Hospital. 

 The enquiry was prosecuted no further, until lately, when it 

 was resumed ; and its results, imperfect as they are, I now 

 communicate, with a view of inducing attention to this point, 

 from those whose pursuits or opportunities may lead them to 

 extend the scanty details I can as yet furnish. 



