[ 3 °° n 
XX. An Account of a Membrane in the Eye , now first described. 
By Arthur Jacob, M. D. Member of the Royal College of 
Surgeons in Ireland , Demonstrator of Anatomy and Lecturer 
on Diseases of the Eye in the University of Dublin . Commu- 
nicated by James Macartney, M. D. F. R. S. 
Read July 1, 1819. 
Anatomists describe the retina as consisting of two por- 
tions, the medullary expansion of the nerve, and a membra- 
nous or vascular layer. The former externally, next to the 
choroid coat, and the latter internally, next to the vitreous 
humor.* All however, except Albinus and some of his 
disciples, agree, that the nervous layer cannot be separated 
so as to present the appearance of a distinct membrane, 
though it may be scraped off, leaving the vascular layer per- 
fect. That the medullary expansion of the optic nerve is 
supported by a vascular layer, does not I think admit of 
doubt ; but it does not appear that Albinus was right in sup- 
posing that the nervous layer can be separated in form of a 
distinct membrane, though shreds of a considerable size may 
be detached, especially if hardened by acid or spirit. 
Exclusive of these two layers, I find that the retina is 
covered on its external surface by a delicate transparent 
• Ru ysch. Epist. Anat. Prob.xiii. Albinus, Annot. Acad. lib. iii. cap. xiv, 
Haller, Elem. Phys. T. v. lib. xvi. sect. 2. Zinn. Descrip. Anat. Oculi. cap. iii. 
sect. iii. Sabatier, Boyer, Charles Bell, Cuvier, &c. Sec. 
