KYE. 



187 





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 Unit 

 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. 



" 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 inves- 

 tigation on this subject would lead to a dis- 

 russimi 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 foetus 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 tlw 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 Ilaller 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. If 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 clown in large quantity. Or if the 

 cornea and iris be removed anteriorly, and the 

 vitreous humour and lens withdrawn, the retina 

 m.i\ 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 Mailer to snow. 



" Besides being connected to the retina, 1 find 

 that the membrane is also attached to the cho- 

 roid coat, apparently by fine cellular sub 

 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 coat, 

 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 for<vp>, 

 pulling it gently from the choroid, when it will 

 appear beyond a doubt that there is a connec- 

 tion between them. 



" Let us contrast this account of the matter 

 with the common one. The retina, a mem- 

 brane of such delicacy, is described as being 

 intruded between the vitreous humour and 

 choroid, from the optic nerve to the ciliary 

 processes, being merely laid between them, 



