246 DR KNOX on the Philosophical Anatomy 



ty, which simply extracting the cataract through an opening in 

 the cornea, must have over couching, or, indeed, over any other 

 operation for the destruction of the cataract, where the needle 

 or knife comes in contact with the canal of Petit and the inter- 

 nal ciliary processes. I recommend this consideration strongly 

 to the attention of the practical surgeon ; for if my memory fails 

 not, the surgeon, in performing the operation of couching, en- 

 deavours to pass his couching-needle just anterior to the termi- 

 nation of the retina. Now, this is precisely the spot which ana- 

 tomy teaches us to avoid, because it is here that the texture is 

 most complex, the vascularity greatest, and on the integrity of 

 which the vitreous body depends for its support. 



In my former paper on this subject, I ventured to suggest 

 that the marsupium in the eyes of birds, is actually the tunica 

 vasculosa observed in the retina of man and other animals, or at 

 least the vascular part of that tunic ; that in place of being ex- 

 panded over the inner surface of the retina, the vessels were col- 

 lected into a group or bundle, and in this manner pierced the 

 vitreous humor, on their way towards its anterior part. I was 

 ignorant, at that time, of the extreme vascularity of the inter- 

 nal ciliary processes. This fact places the subject in a new light, 

 and proves that the marsupium in birds and fishes is the substitute 

 for the internal ciliary processes *. 



The preparations which I have the honour to submit to the 

 consideration of the Society, prove the mode of formation of 

 the Canal of Petit to be different from what has been ge- 

 nerally described. I have found, that near the point where the 



* I do not mean that there are no branches proceeding to the retina in birds, but 

 merely that they are so small as entirely to escape notice, and no longer constitute a 

 network on the inner surface of the retina, as in the mammalia. 



