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 pores 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 Siimmerring, 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, I 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 off, 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 he 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 cause I leave the retina uninjured, and present ing the appearance described by anatomists, especially the yellow spot of Siiemmerring, which is never seen to advantage until this membrane be removed : and hence it is that conformation, as well as the fibrous structure of the retina in some animals, become better marked from remaining some time in water, by which the membrane I speak of is de tached.