( S23 ) 
Art. Observations on the Structure and Functions (^tlm 
Canal of Petit., and the Marsupium.Nigruin, or peculiar Vas- 
cular Tissue traversing the vitreous humour in the Eyes of 
Birds., Fishes, and Reptiles. Communicated in a Letter to 
Dr Biiewster, by Robert Knox, M. D. F. R, S. E. &c, &c« 
My Dear Sir, 
It may perhaps be in your recollection, that in the Essay 
which I had the honour to read before the Royal Society of 
Edinburgh, on the Comparative Anatomy of the Eye in June 
1823, and in the several memoirs connected with the same sub- 
ject, which I have subsequently submitted to that learned body, 
I had not been able to determine, with precision, the nature 
, and functions of pecten or marsupium nigrum, iowod. inva- 
riably, I presume, in the eyes of birds. After I had ascertain- 
ed (what indeed had been done previously), that it is very vas- 
cular, and generally dark coloured, that it contains a quantity 
of loose cellular tissue, and no nerves, and that it seldom pro- 
ceeds so far as the lens, passing only a certain way into the vi- 
treous humour, I found, that all the questions of high physio- 
logical interest, connected with this peculiar structure, still re- 
mained unanswered. For example, there did not appear any sa- 
tisfactory or sufficient reason why this vascular tissue was placed 
in the centre of the vitreous humour ; neither had any one shewn 
its true functions. To clear up these points, I repeated a num- 
ber of my dissections, and arranged a series of preparations of 
the vascular tissues of the eye in birds and quadrupeds, illus- 
trative of some of my opinions. These preparations, together 
with an Essay on the subject, I had the honour to submit to the 
Royal Society on the 15th of March. The following very brief 
Abstract will, I trust, enable the readers of the Philosophical 
Journal to comprehend the general views I have adopted rela- 
tive to the subject 
* The Memoir is not confined altogether to an inquiry into the functions of 
the Marsupium ; several other points relative to the comparative anatomy of. the 
JEye are briefly discussed. 
