186 



EYE. 



brane, of such strength, that when detached, it 

 may be moved about in water, and freely ex- 

 amined without breaking. It adheres so firmly 

 to the hyaloid membrane of the vitreous hu- 

 mour in the fresli eye, that it cannot be sepa- 

 rated entire, and the medullary fibres adhere 

 so closely to its external surface, that they can- 

 not be detached at all in the form of a distinct 

 membrane. To demonstrate the vascular layer, 

 the sclerotic should be carefully removed, leav- 

 ing a portion of the optic nerve freed from its 

 sheath ; the choroid should then also be re- 

 moved under water, by tearing it asunder with 

 a pair of forceps in each hand. The vitreous 

 humour, covered by the retina only, should 

 then be allowed to remain about two days in 

 the water, at the end of which time the me- 

 dullary layer softens and separates into flakes, 

 which may be scraped from the vascular layer 

 beneath by passing the edge of a knife gently 

 over it, after which the vascular layer may be 

 detached by careful management, and sus- 

 pended in a bottle from the optic nerve. 



The retina is supplied with blood from the 

 ophthalmic artery, a small branch of which 

 penetrates the optic nerve at a short distance 

 from the back of the eye, and proceeds through 

 its centre until it arrives at the retina. The 

 hole in the centre of the nerve, through which 

 it passes, was formerly called the porus opticus. 

 Arrived at the retina, the vessel, under the 

 name of the central artery of the retina, divides 

 into two branches, which surround the foramen 

 of Sommerring, and sending ramifications in 

 every direction, terminate by encircling the an- 

 terior margin. Besides the branches which 

 carry red blood, the central artery probably 

 furnishes a transparent branch to the centre of 

 the vitreous humour, as such a branch running 

 on to the back of the crystalline lens, may be 

 injected in the eye of the foetus, and a transpa- 

 rent production from the central artery into the 

 vitreous humour may be observed in the eyes 

 of oxen and other large animals. The arteries 

 of the retina supply the vitreous, humour with 

 blood, as no other source exists, except from 

 the ciliary processes of the choroid, which, 

 being buried in the hyaloid membrane, most 

 probably furnish vessels to the anterior part, 

 and in dissecting the vascular layer above de- 

 scribed, in which the vessels ramify, it is found 

 to adhere to the hyaloid membrane by points 

 along the course of the vessels, which points, it 

 is reasonable to believe, are small branches. 



As the medullary or cerebral fibres of the 

 retina are sustained on the inside by the vascu- 

 lar layer above described, they are also protected 

 on the outside by another membrane, which 

 separates them from the inner surface of the 

 choroid. This is the membrane which I des- 

 cribed in a communication in the Philosophical 

 Transactions in 1819, and as I cannot give a 

 more intelligible account of it than that there 

 contained, 1 venture to introduce it here. 



" Anatomists describe the retina as consisting 

 of two portions, the medullary expansion of 

 the nerve, and a membranous or vascular layer. 

 The former externally, next to the choroid coat, 

 and the latter internally, next to the vitreous 



humour. 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 otf, leaving the vascular layer 

 perfect. 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 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 two layers, I find that 

 the retina is covered on its external surface by 

 a delicate transparent membrane, united to it 

 by cellular substance and vessels. This struc- 

 ture, 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 perfect membrane, apparently of 

 the same nature as that which lines serous cavi- 

 ties. I cannot describe it better, than by detailing 

 the method to be adopted for examining and dis- 

 playing it. Having procured a human eye, 

 within forty-eight hours after death, a thread 

 should be passed through the layers of the cor- 

 nea, by which the eye may be secured under 

 water, by attaching it to a piece of wax, previ- 

 ously 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 ex- 

 perienced eye may perceive, that this is not 

 the appearance usually presented by the retina ; 

 instead of the blue-white reticulated surface of 

 that membrane, a uniform villous structure, 

 more or less tinged by the black pigment, pre- 

 sents 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 mem- 

 brane 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 affords. 

 If a small opening be made in the membrane, 

 and the blunt end of a probe introduced be- 

 neath, 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 

 sometimes so strong as to support small glo- 

 bules of quicksilver dropped between it and 

 the retina, which renders its membranous na- 

 ture 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 de- 

 tach, is proved by the most superficial exa- 

 mination ; first, because it is impossible to 

 separate that part of the retina, so as to present 

 the appearance I mention ; and, secondly, be- 



