VISION. 19 



this apparent discrepancy between facts and the text, 

 St. Chrysostom proposed to read "vultures" for 

 " eagles," in the passages both in Matthew and Job *. 

 Aldrovand, we think, has given the only judicious 

 solution of the difficulty t by referring to a very com- 

 mon oriental species (Gypaetus barbatus, STORR), 

 which was remarked by Aristotle to be similar in 

 form to the eagle, but had more the habits of the 

 vulture J. 



Besides the nictitating membrane in the eye of 

 birds already described, which is not altogether 

 peculiar to them, there is another singular part of 

 the organ whose use has not hitherto been clearly 

 ascertained. It is called by the French Acade- 

 micians the purse (marsupium) , and the comb 

 (pecten plicatum). It arises in the back of the 

 eye, and proceeding apparently through a slit in the 

 retina, it passes obliquely into the vitreous humour, 

 where it terminates, reaching in some species to the 

 capsule of the lens. Numerous blood-vessels run in 

 the folds of the membranes which compose it, and 

 the black pigment by which it is covered suggests 

 the idea that it is chiefly destined to absorb the rays 

 of light when they are too strong or dazzling: if 

 this be the fact, it may serve the eagle in good stead 

 when gazing, if he ever do so, on the s.un. It is the 

 opinion of others, that it serves to assist in producing 

 the internal changes of the eye ; but this has been 

 opposed by Crampton, who has shown that the 

 changes in question, at least in the ostrich and 

 several large birds, are produced by a peculiar cir- 

 cular muscle in the eyeball ||. 



* Chrysost. Horn. xlix. f Ornithologia, i. 20. 



I Hist. Anim. ix. 32. 

 Blumenbach, Comp. Anat. .290. 

 || Thomson's Annals, March, 1813. 



