of the Canal of PETIT. 245 



vitreous humor. Hence we see the reason why the marsupium 

 does not extend in general as far as the capsule of the lens, an 

 extent which its functions do not in the least necessitate. The 

 same structure exists in certain fishes, and apparently for the 

 same reason ; but, on account of the extreme difficulty of inject- 

 ing the bloodvessels in these animals, I am unable to speak from 

 so great a number of observations as I could have wished *. 

 The colour of the marsupium does not seem to be of so great 

 moment as one would at first suppose. It is very generally 

 dark-coloured throughout in birds, whilst in fishes it is most ge- 

 nerally coloured only in a small portion, and is quite transparent 

 near its base :jl. 



I suppose the internal ciliary processes to be comparatively 

 less vascular in man than in many other animals ; stiU we know 

 that the canal of Petit is very distinct, and no doubt has its pa- 

 rietes well supplied with bloodvessels. The appearances pre- 

 sented by these parts impressed on my mind the great superiori- 



* It would seem that in some fishes the vitreous humor is supplied by vessels in 

 both ways +, i. e. a branch of the central artery of the retina passes directly into the 

 centre of the vitreous humor, whilst the remaining branches pass into the same, 

 but more anteriorly through the medium of the marsupium. They anastomose about 

 the centre of the vitreous humor. We owe this fact to HALLER. It seems to 

 me quite conclusive as to the real functions of the marsupium in fishes ; yet this 

 great anatomist calls the marsupium the sustentaculum of the lens, and says that in 

 birds it carries the blood to the crystalline humor. 



J The coloration of the iris, in itself a curious subject, is rendered more so by 

 several facts, which do not very readily admit of explanation ; such as the iris being 

 differently coloured in the two eyes of the same individual, an appearance I have 

 seen not unfrequently. QUINTUS CUKTIUS relates, that the eyes of ALEXANDER 

 the Great presented this appearance. TENON gives a very good engraving of an iris 

 on which there may be seen the letter T very distinctly, 

 f- Element. Physiol. t. v. p. 391. 



