DR KNOX on the Comparative Anatomy of the Eye. 69 



and the ciliary nerves are numerous, and send branches round 

 the iris, in the same way as in birds *. 



In most fishes there is no true or moveable iris. In the 

 mammalia its relations appear complex ; but they may be unra- 

 velled, by comparing the structure with birds. I shall here de- 

 scribe very briefly its anatomy in the latter animals, and the 

 mode in which it may be best displayed. If we select the eye of 

 the larger birds, we may, unaided by glasses, perceive that the 

 pigmentum nigrum, covering the uvea, is inclosed in a delicate 

 and distinct membrane, and that this membrane proceeds from 

 the anterior part of the ciliary processes. The expansion of the 

 inner membrane of the choroid into the ciliary bodies, and then 

 into the uvea, is also most distinctly observed ; the formal termi- 

 nation of the middle layer, or true iris, in a brownish cellular 

 membrane, shewing clearly that it is a peculiar body, and not 

 a continuation of any other membrane ; and the connection this 

 cellular membrane has with the cornea, and with the tendinous 

 expansion of the ciliary muscle. 



* I ought to have inserted, in this part of the observations, the result of my in- 

 quiries into the retina, its distribution, nature, &c. ; but at the time this memoir was 

 presented, I did not deem the researches sufficiently complete, or fitting to be sub- 

 mitted to the Royal Society of Edinburgh. But having repeated some of the dissec- 

 tions, and (the opportunity having presented itself) extended the researches, I made 

 the important, and most unexpected discovery, that Onejbramen centrale of the re- 

 tina, generally called the Foramen of S<EMM BEING, is not confined to the eye of Man, 

 and of some Quadrumanous Animals, whose organisation somewhat approaches him, 

 but is extended to the class Reptiles, contrary to the opinion of all comparative anato- 

 mists. The details of this very singular discovery are reserved for the Memoirs 

 of the Wernerian Society, and the fact is simply announced in this place, to make my 

 other observations more complete. " The foramen centrale of the retina exists in 

 many lizards, as the superciliosa, scutata, striata, and Calotes, and is in them compa- 

 ratively much more developed than in Man ; but it is wanting in the lizards called 

 gecko, mabuya," &c. See Memoirs of the Wernerian Society, Vol. v. Part 1. 



