234 DR KNOX on the Philosophical Anatomy 



inner membrane passes on uninterruptedly *. So far, then, it 

 would appear, that the transparent point of SOEMMERING is occa- 

 sioned by a deficiency of the pulpy portion of the retina. 



We thus get rid of a war of words, which probably would 

 have arisen from the fact of the internal tunic being continuous, 

 which was never questioned. 



The opacity assumed by the retina after immersion in spirits, 

 is chiefly owing to its pulpy layer ; and as this is wanting at the 

 foramen centrale, or directly in the axis of vision, we readily 

 perceive why the appearance should become more distinct after 

 the eye has been immersed in spirits. 



In answer to those who argue, that, as the retina is perfectly 

 transparent during life, so the foramen centrale can be said to 

 exist only some time after death, when the nervous membrane, 

 becoming everywhere opaque, excepting in the line of the axis 

 of vision, permits our seeing the choroid at this particular spot ; 

 I would reply, That such might be the case were it only a trans- 

 parent point ; but it has been already shewn, that the pulpy por- 

 tion of the membrane is absent. I have, moreover, examined the 

 eye-ball in a great number of animals immediately after death, 

 and never found the retina to be absolutely transparent (except- 

 ing when dried), but uniformly of a bluish colour, and very 

 slightly, though in general visibly opaque. The bluish cast I 

 attribute to the subjacent choroid. This remark I have made 

 on the eyes of several of the domestic animals, as oxen, sheep, 

 horses, dogs, &e ; on many birds, and particularly on fishes ; 

 and, lately, on man himself. For this I am indebted to Dr 

 MONRO, who, with his accustomed liberality, permitted me 

 to examine the eyes of a man who was executed in this 



* As I am assured that vascularity is not the essential character of this mem- 

 brane, I shall prefer calling it by the name of the inner membrane of the retina : the 

 reasons for so doing will be given afterwards. 



