Account of a Memlrane in the Eye now first descriled. 127 



of alkali, by vegetable suppuration, to that which is effected by 

 combustion. 



Limine exists in many ancient products of the vegetable king- 

 dom. I found it long ago in making the analysis of an earth 

 found in the roots of an old tree*; but I did not then examine 

 all its properties. It even appears that the soluble part of cer- 

 tain earths, which have been ranked with the extractive, is formed 

 of ulmine and ammonia. I have also found ulmine in great abun- 

 dance in turf as well as in a variety of ligneous earths. It makes, 

 doubtless, too, a constituent part of soot j but I have found it 

 impossible to produce ulmine with oil. 



XVII. An Account of a Membrane in the Eye, now first de- 

 scribed. By Arthur Jacob, M.D. Member of the Royal 

 College of Surs^eons in Ireland, Demonstrator of Anatomy 

 and Lecturer on Diseases of the Eye in the University of 

 Dublin. Communicated by James Macartney, M.D. 

 F.R.S.f 



xiSAToM.'STS describe the retina as consisting of two portions, 

 the medullary expansion of the nerve, and a membranous or vas- 

 cular layer. The former externally, next to the choroid coat, 

 and the latter internally, next to the vitreous humour X. 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, leavhig the vascular layer perfect. That the medullary ex- 

 pansion of the optic nerve is supported by a vascular layer, does 

 not I think admit of doubt ; but it does nof appear that Albinus 

 was right in supposing 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 twalaveis, I find thai the retina is covered 

 on its external surface by a delicate transparent membrane, united 

 to it by cellular substance and vessels. This structure, not 

 hitherto noticed by anatomists, I first observed in the spring of 

 the last year, and have since so frequently {demonstrated, as to 

 leave no doubt on my mind of its existence as a distinct and pti - 

 feet membrane, apparently of the same nature as that which lines 

 serous cavities. I cannot describe it better, than by detailing the 



• Annnlet de Ch'imie, torn. Ixi. p. U'l. 



t From )1iilo3f)|)liical Transadioiis for 1H19, Part II. 



\ Uuyscli. Kpist. Anat. I'rob. xiii. Albinus, Annot. Acad. lib. iii.cap. xiv. 

 Haller, Klem. I'hys. t. v. lib. xvi .<ect. 2. Zinn. Dcscrip. Anal. Oculi, Cdp.iii. 

 Met. iii. Sabaticr, Boyci, Charles bell, Cuvicr, &c. 



method 



