8.‘24 Dr Knox on the Structure and Functions 
1. Of the Retina. 
The result of several inquiries which I have made into the 
structure of this membrane, with the microscope and otherwise, 
is, that the foramen centrale of the retina is a perforation of the 
pulpy membrane of the retina, which perforation does not ex- 
tend to the internal membrane. The functions of this most ex- 
traordinary structure in the eyes of man, quadrumanous ani- 
mals, and certain reptiles, are exceedingly mysterious, and have 
become, if possible, still more obscure, by the discovery of its 
presence in the chameleon 
The anterior termination of the retina is shewn to be nearly 
as described in my former observations ; but the capsules form- 
ing the canal of Petit, have been described with a greater de- 
gree of minuteness, and, it is hoped, of accuracy. The inter- 
nal ciliary processes {Zonula ciliarisoi Zinn.) have been proved 
to be extremely vascular, and to communicate, by anastomosing 
vessels, with the tunica vasculosa retinrt \ these structures exist 
in their highest degree of development in man and quadrupeds, 
or, generally speaking, in the mammalia. In birds and certain 
fishes, another arrangement takes place ; the vessels entering in- 
to the composition of the tunica vasculosa retinse, and of the in- 
ternal ciliary processes of the mammalia, are transferred to the 
marsupium or pecten : the inner membrane of the retina in these 
animals is no longer vascular, and seems hardly to exist as a 
membrane ; the complicated structure forming the capsules and 
canal of Petit has disappeared, and become so completely ru- 
dimentary, as to be denied by some anatomists. It is inferred 
fr,om these and other data, that these are corresponding and 
analogous parts in the two classes of aninials, viz. that the pec- 
ten in birds is the vascular portion of the retina, and of the ca~ 
rial of Petit, found only in the mammalia, the functions of which 
seem to be to secrete and nourish the vitreous humour. 
S. Of the Membrane of Jacob. 
Some speculations relative to this membrane, contained in the 
* See a Memoir, entitled, “ An Account of the Discovery of the Foramen 
Centrale Hetinse in the Eyes of certain Reptiles. By R, Knox, M. D.” Mem. Wern’, 
Soc. VoL V. Part i. ; also Vol. ix. p. 358. of this Journal. 
