136 Peculiar Structure in the Eyes of Fishes. 



rience, (not "imaginative theorists"), have convinced, (not 

 "persuaded") themselves "are there." I shall then be happy 

 to give him an opportunity of following a candid example, by 

 enabling him to correct his own errors, in acknowledging, as 

 I point them out to his sight, the existence of "curved strata 

 and breccia of Purbeck stone," at Durlstone Head ; — of Has- 

 tings sands, and embedded fossils, in the cliffs of Swanwich 

 Bay ; — of green sand, &c. &c. at Punfield, and between it 

 and Ballard Point ; — and of all the other phenomena which I 

 have stated to characterise these interesting sections. 



Stanley Green, 6th Dec. 1837. 



Art. V. Some account of a peculiar Structure in the Eyes of 

 Fishes. By John Dalrymple, Esq. Lecturer on Surgery at Sy- 

 denham College. 



If it be considered necessary that animals, living in atmo- 

 spheric air, should possess some description of mechanism, 

 by which the focal adjustment of the eye can be regulated 

 for near or distant vision, it may be presumed that those 

 which inhabit an aqueous medium, should be gifted with some 

 apparatus for a like purpose. 



Land animals have been thought to possess this power of 

 adaptation, — 1st, by an alteration of the convexity of the cor- 

 nea, through the action of the straight muscles of the globe ; 

 a position which, though argued with considerable ingenuity 

 by Sir E. Home, in the Croonian Lectures, has yet found, 

 and is likely to find, but few advocates, except, perhaps, in 

 relation to some birds. 



2ndly, by an action of the ciliary processes ; their extreme 

 vascularity rendering it probable that something analogous to 

 erectility of tissue may serve this end. 



3rdly, by the action of the iris itself, whether it be erectile 

 or muscular ; for it is well ascertained that contraction and 

 dilatation take place, in exact accordance with the proximity or 

 distance of the object viewed. Persons in whom the pupil has 

 been widely dilated by the action of Belladonna, are much more 

 capable of viewing distant than near bodies distinctly. Other 

 circumstances, connected with these states of contraction and 

 dilatation, render it inconclusive whether the iris possesses 

 the sole power in regulating the focal relations of the eye. 



In birds and reptiles there is superadded a new organ, the 

 pecten or marsupium ; which, while it bears, in appearance 

 and structure, a resemblance to the ciliary processes, yet, 



