of the Canal of PETIT. 233 



1 hold this opinion to be altogether incorrect *, as applied to 

 the pulpy or true retina, but readily admit that the perforation 

 does not extend to the whole of the membrane usually called 

 Retina. The retina is composed, in most animals, of at least two 

 membranes f, viz. an external or pulpy layer, and an internal, (ge- 

 nerally vascular), described improperly under the name of tunica 

 vasculosa Retina. Air blown in betwixt the retina and choroid in 

 the situation of the Membrane of Jacob, cannot pass into the 

 chamber of the eye containing the vitreous humour, because it 

 is arrested in its passage through the foramen centrale by the in- 

 ternal tunic of the retina. When I discovered that the foramen 

 centrale of the retina was not peculiar to man and the quadru- 

 mana, as all anatomists before me believed ; but that, on the con- 

 trary, it was extremely developed in the cameleon, and in certain 

 lizards somewhat allied to the cameleon, I judged it a favour- 

 able opportunity for re-examining the subject with great atten- 

 tion. The structure was viewed under a good microscope, aided 

 by a strong light, and submitted to a number of gentlemen well 

 qualified to judge of such matters. Now, on this subject, there 

 was but one opinion, viz. that the pulpy part of the retina in the 

 situation of the foramen of SCEMMERING is absent, but that the 



* See the facts stated in my " Account of the Discovery of the Foramen Cen- 

 trale of the Retina, in the eyes of certain Reptiles,'" published in the Memoirs of 

 the Wernerian Society, vol. 5. p. 1. 



f I conclude that even when the retina has no longer a tunica vasculosa, as we 

 find to be the case in birds, there still exist two layers, vascularity not being the essen- 

 tial character of either. Dissections by HALLER seem to confirm this idea; his words 

 are, " Lamina ergo hie in retina interior fibrosa est (in piscibus), et alia exterior 

 pulposa."" But he allows that it is exceedingly difficult (he might have added impos- 

 sible) to demonstrate this structure in birds ; it must therefore be simply inferred by 

 analogy . 



I The Lacerta superciliosa, scutata, &c. 



Gg 2 



