now Jirsl described. 129 



of the optic nerve, and the termination of the retina anteriorly, 

 to which it is my intention to retmii 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 delicate, 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 extra- 

 ordinary 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 pa'js a probe under it, and thus convey the vi- 

 treous 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 par- 

 tially separated from tlie 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 l>iack choroid coat. In the bird, it pre- 

 sents a rich yellow brown tint, and when raised, the blue retina 

 presents .itself beneath : in animals of this class, hov^-ever, it is 

 difficult to separate it to any extent, though I can detach it in 

 small portions. In fishe«, the structure of this menibrane is pe- 

 culiar and curious. It has been already described as the medul- 

 lary layer of the retina by Haller and Cuvier*, but I think incor- 

 rectly, as it does not present any of the characters of nervous 

 structure, and the retina is found j)erfect beneath it. If the 

 sclerotic coat lie removed behmd, with the choroid coat and 

 gland so called, the l)lack pigment is found resting upon, and 

 attached to, a soft friable thick lleeoy structure, which can only 

 be detached in small portions, as it breaks when turned down in 

 large (juantity. 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 and the vitreous humour and lens withdrawn, the retina may be 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 remains attached to the cho- 

 roid coat, its inner surface not tinged by the black pigment, but 

 presenting a clear white, tiot unaptly compared by Mailer to 



licsides being connected to the retina, I find that the mem- 

 brane is :iNo attached to the choroid coat, apparently by fine 

 cellular substance and vessels; but its cotmecti(m with the retina 

 being stronger, it generally remains attached to that membrane, 



* Klemt-nt. Fliys. t. v. lili. xvi. sect. ii. Cuvicr, Levmis d'Aimt. Comj). t. ii. 



p. 41fy. 



Vol,::«.No. 2G2. /'V/'. 1S20. I though 



