" The extent and connections of this mem brane are sufficiently explained by saying, that it covers the retina from the optic nerve to the ciliary processes. To enter into farther hives tigation on this subject would lead to a dis cussion respecting the structure of the optic nerve, and the termination of the retina an teriorly, to which it is my intention to return at a future period.
"The appearance of this part I find to vary in the different classes of animals and in man, according to age and other circumstances. In the fcetlis of nine months it is exceedingly de licate, and with difficulty displayed. In youth it is transparent, and scarcely tinged by the black pigment. In the adult it is firmer, and more deeply stained by the pigment, which sometimes adheres to it so closely as to colour it almost as deeply as the choroid coat itself; and to those who have seen it in this state, it must appear extraordinary that it should not have been before observed. In one subject, aged fifty, it possessed so great a degree of strength as to allow me to pass a probe under it, and thus convey the vitreous humour covered by it and the retina from one side of the basin to the other ; and in a younger subject I have seen it partially separated from the retina by an effused fluid. In the sheep, ox, horse, or any other individual of the class mammalia which I have had an opportunity of examining, it presents the same character as in man ; but is not so much tinged by the black pigment, adheres more firmly to this retina, is more uniform in its structure, and presents a more elegant appear ance when turned down over the black choroid coat. In the bird it presents a rich yellow brown tint, and when raised, the blue retina presents it self beneath; in animals of this class, however, it is difficult to separate it to any extent, though I can detach it in small portions. In fishes, the struc ture of this membrane is peculiar and curious. It has been already described as the medullary layer of the retina by Haller and Cuvier, but I think incorrectly, as it does not present any of the characters of nervous structure, and the retina is found perfect beneath it. I the scle rotic coat be removed behind, with the choroid coat and gland so called, the black pigment is found resting upon, and attached to, a soft friable thick fleecy structure, which can only be detached in small portions, as it breaks when turned down in large quantity. Or if the cornea and iris be removed anteriorly, and the vitreous humour and lens withdrawn, the retina may be pulled from the membrane, which re mains attached to the choroid coat, its inner surface not tinged by the black pigment, but presenting a clear white, not unaptly compared by Haller to snow.
" Besides being connected to the retina, I find that the membrane is also attached to the cho roid coat, apparently by fine cellular substance and vessels; but its connection with the retina being stronger, it generally remains attached to that membrane, though small portions are sometimes pulled off with the choroid coat. From this fact I think it follows, that the accounts hitherto given of the anatomy of these parts are incorrect. The best anatomists de scribe the external surface of the retina as being merely in contact with the choroid coot, as the internal with the vitreous humour, but both totally unconnected by cellular mem brane, or vessels, and even having a fluid secreted between them : some indeed speak loosely and generally of vessels passing from the choroid to the retina, but obviously not from actual observation, as I believe no one has ever seen vessels passing from the one membrane to the other. My observations lead me to conclude, that wherever the different parts of the eye are in contact, they are con nected to each other by cellular substance, and, consequently, by vessels; for I consider the failure of injections no proof of the want of vascularity in transparent and delicate parts, though some anatomists lay it down as a cri terion. Undoubtedly the connection between these parts is exceedingly delicate, and, hence, is destroyed by the common method of ex amining this organ ; but I think it is proved in the following way. I have before me the eye of a sheep killed this day, the cornea secured to a piece of wax fastened under water, and the posterior half of the sclerotic coat carefully removed. I thrust the point of the blade of a pair of sharp scissors through the choroid coat into the vitreous humour, to the depth of about an eighth of an inch, and divide all, so as to insulate a square portion of each membrane, leaving the edges free, and consequently no connection except by surface; yet the choroid does not recede from the mem brane I describe, the membrane from the retina, nor the retina from the vitreous humour. I take the end of the portion of choroid in the forceps, turn it half down, and .pass a pin through the edge, the weight of which is in sufficient to pull it from its connection. I se parate the membrane in like manner, but the retina I can scarcely detach from the vitreous humour, so strong is the connection. The same fact may be ascertained by making a transverse vertical section of the eye, removing the vitreous humour from the posterior seg ment, and taking the retina in the forceps, pulling it gently from the choroid, when it will appear beyond a doubt that there is a connec tion between them.