302 
Dr. Jacob’s account of 
light, and is thus displayed to great advantage ; or it is some- 
times so strong as to support small globules of Quicksilver 
dropped between it and the retina, which renders its membra- 
nous nature still more evident. If a few drops of acid be 
added to the water after the membrane has been separated, 
it becomes opaque and much firmer, and may thus be pre- 
served for several days, even without being immersed in 
spirit. 
That it is not the nervous layer which I detach, is proved 
by the most superficial examination ; first, because it is im- 
possible to separate that part of the retina, so as to present 
the appearance I mention ;* and, secondly, because I leave 
the retina uninjured, and presenting the appearance described 
by anatomists, especially the yellow spot of Soemmerring, 
which is never seen to advantage until this membrane 
be removed : and hence it is that that conformation, as well 
as the fibrous structure of the retina in some animals, 
becomes better marked from remaining some time in water, 
by which the membrane I speak of is detached. 
The extent and connections of this membrane are suffi- 
ciently explained by saying, that it covers the retina from 
the optic nerve to the ciliary processes. To enter into far- 
ther investigation on this subject, would lead to a discussion 
respecting the structure of the optic nerve, and the termina- 
tion of the retina anteriorly, to which it is my intention to 
return at a future period. 
The appearance of this part I find to vary in the different 
classes of animals and in man, according to age and other 
circumstances. In the foetus of nine months it is exceed- 
* See Haller, Zinn, &c. loc. cit. 
