128 Account of a Membrane in the Eye 



method to be adopted for examining and displaying it. Having 

 procured a human eye, within forty-eight hours after death, a 

 thread shouhl be passed tiirongh the layers of the cornea, by 

 which the eye may be secured under water, by attaching it to a 

 piece of wax, previously fastened to the bottom of the vessel, the 

 posterior half of the sclerotic having been first removed. With a 

 pair of dissecting forceps in each hand, the choroid coat should 

 be gently torn open and turned down. If the exposed surface be 

 now carefully examined, an experienced eye may perceive, that 

 this is not the appearance usually presented by the retina ; in- 

 stead of the blue-white reticulated surface of that membrane, a 

 uniform villous structure, more or less tinged by the black pig- 

 ment, presents itself. If the extremity of the ivory handle of a 

 dissecting knife be pushed against this surface, a breach is made 

 in it, and a membrane of great delicacy may be separated and 

 turned down in folds over the choroid coat, presenting the most 

 beautiful specimen of a delicate tissue which the human body af- 

 fords. If a small opening be made in the membrane, and the 

 blunt end of a probe introduced beneath, it may be separated 

 throughout, without being turned down, remaining loose over 

 the retina; in which state if a small particle of paper or globule 

 of air be introduced under it, it is raised so as to be seen against 

 the light, and is thus displayed to great advantage ; or it is some- 

 times so strong as to support small globules of i|uicksilver dropped 

 between it anti the retina, which renders its membranous 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 preserved 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 impossible 

 to separate that part of the retina, so as to present the appear- 

 ance I mention * ; and, secondly, because I leave the retina un- 

 injured, 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 re- 

 tina in some animals, becomes better marked from remaining 

 seme time in wate'r, by which the membrane I speak of is de- 

 tached. 



The extent and connections of this membrane are sufficiently 

 explained by saving, that it covers the retina from the optic nerve 

 to the ciliary processes To enter into further investigation on 

 this subject, would lead to a discussion respecting the structure 



* 6ee Haller, Zinn. -&c. loc. cit. 



of 



