a membrane in the eye. 
S°3 
ingly delicate, and with difficulty displayed. In youth it istrans- 
parent, 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 
humor 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 the 
retina, is more uniform in its structure, and presents a more 
elegant appearance when turned down over the black cho- 
roid coat. In the bird, it presents a rich yellow brown tint, 
and when raised, the blue retina presents itself 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 structure 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. If the sclerotic 
coat be removed behind, with the choroid coat and gland so 
* Element. Phys. T. v, lib. xvi. sect. ii. Cuvier. Lejons d’Anat. Comp. 
T. ii. p. 419. 
