64 DR KNOX on the Comparative Anatomy of the Eye. 



becomes incorporated with a membrane. This takes place im- 

 mediately anterior to the termination of the retina. Though 

 the iris, and all the choroid coats, be removed, it is evident, that 

 there is still a very thin transparent membrane betwixt you and 

 the retina, connecting this latter with the membranous pigmen- 

 tum nigrum, and extending forwards to the edge of the pupil. 

 When the retina is pulled towards the optic nerve, this mem- 

 brane is drawn along with it ; when detached, the dark coloured 

 membrane assumes a fimbriated edge. The termination of the 

 retina is very precise and determinate ; the dark coloured mem- 

 brane adheres firmly to the subjacent hyaloid, but may be rea- 

 lily separated, for a certain distance, by commencing at the in- 

 ner edge of the pupil ; but on approaching the terminating 

 edge of the retina, the union becomes very intimate, and a 

 junction of all these membranes is very evident, by means of a 

 circular firm line, visible even to the naked eye. I have not ob- 

 served any adhesion of this dark coloured membrane to the 

 crystalline, and am, indeed, certain of the contrary. At its an- 

 terior edge, it quits the hyaloid, and proceeds to form that dark 

 membrane lining the iris, whilst the hyaloid, clear and transpa- 

 rent, advances forwards to inclose the lens. 



The ease with which the retina may be separated from the 

 black membrane, shews that it lies external to the process of the 

 membrane connected with the hyaloid ; and this becomes evi- 

 dent, by observing the retina preserve, on being detached, its 

 anterior formal outline, or termination, and by the black mem- 

 brane still adhering to the hyaloid, which appears quite smooth 

 and continuous ; the anterior part of the retina is then, as it 

 were, inclosed between two folds of the membrane of the pig- 

 mentum nigrum, the exterior of which, no doubt, was described 

 long ago ; vessels unite it with the internal part of the iris, to 

 which, at these points, it becomes firmly attached. 



