of the Canal of PETIT. 239 



Both layers are vascular ; the external one remarkably so, re- 

 ceiving innumerable branches, perhaps chiefly veins, from the co- 

 loured ciliary processes which are immediately superincumbent 

 to them, and they anastomose, as has been already stated, with 

 those branches, whether arteries or veins, which we find distri- 

 buted to the inner membrane of the retina in the eyes of most 

 mammiferous animals. But what purpose can this excessive vas- 

 cularity in these internal ciliary processes serve ? I was unable 

 to answer this question satisfactorily to myself, until I ascertain- 

 ed that, in the eyes of birds, the most delicate injections cannot 

 demonstrate any vessels either on the retina or on the internal 

 ciliary processes, and that the canal of Petit can hardly be said 

 to exist in these animals ; that, in short, the whole structure is 

 in them rudimentary ; the vascular or active, and really essential 

 part, being transferred to another organ. 



I need scarcely recal to the remembrance of this learned So- 

 ciety, that it is principally by means of arteries that the various 

 parts of the body are nourished, and that it is through the me- 

 dium of veins and lymphatics that the superfluous parts are re- 

 moved : hence, anatomists have at all times been anxious to de- 

 monstrate the presence of these organs in the various textures 

 of the body, and numerous valuable physiological and pathologi- 

 cal facts, have arisen out of such inquiries. There exist, how- 

 ever, even at present, great differences of opinion as to the 

 source whence the humors of the eye are derived : the mem- 

 brane which some have called the secreting membrane of the 

 aqueous humor, seems to possess no such function * ; whilst to 



* I have shewn in my former paper, that, in the eyes of certain animals, the inner 

 membrane of the cornea may be traced over the anterior surface of the iris, whilst in 

 others we merely infer its presence by analogy. In fishes, in which the first arrange- 

 ment is most distinct, the aqueous humor is very small in quantity. I readily con- 

 fess, that I have not been able satisfactorily to make up my mind as to the source of 

 VOL. X. P. II. H h 



