I>R KNOX on the Comparative Anatomy of the Eye. 65 



Perhaps the most remarkable appendage of the choroid is the 

 marsupium, supposed to exist only in the eyes of birds, but which 

 may be demonstrated in a great many fishes, and, I may add, 

 reptiles. The marsupium is a membranous expansion, sufficient- 

 ly firm, extending from the point at which the optic nerve ex- 

 pands, into the retina, and advancing generally through the 

 centre of the vitreous humour, to be fixed into the posterior and 

 lateral surface of the capsule of the lens, or, more correctly, into 

 that portion of the hyaloid membrane covering the posterior sur- 

 face of the lens. The marsupium is coated inside with a kind 

 of pigmentum nigrum, in some birds for about two-thirds of its 

 course ; in others much more ; the remaining portion, or that by 

 which it is attached to the choroid, is in many birds quite trans- 

 parent, and thus has escaped the notice of the anatomist. This 

 portion is, moreover, exceedingly delicate, and, unless the eye- 

 ball be opened with the greatest caution) is seldom seen. In the 

 eye of the cassowary it is of a brownish colour; In proportion 

 as the true anatomy of the marsupium was unknown, so were its 

 functions deemed mysterious and important ; its presence was 

 supposed confined to the feathered creation, and it thus became 

 associated with the superior vision of birds. But anatomy de- 

 monstrates, that it is simply a continuation of the choroid, that 

 portion being generally transparent by which they are con- 

 nected *. 



I have already observed* that the portion of the choroid 

 which passes over the termination of the optic nerve, is trans- 

 parent, whilst, in those animals in which there is no considerable 

 artery passing through the vitreous humour, but where the ves- 

 sels proceed to the lens immediately under the retina, their 

 vessels will be found to be supported by a delicate transparent 



* Plate III. Fig. 4. 

 VOL. X. P. I. I 



