330 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



quence of a person accidentally shaking the table, that had not been before ob- 

 served. This proved to be a semi-transparent tube, resembling in its coats a 

 lymphatic vessel, rising from the retina, close to the optic nerve, on the temporal 

 side of its insertion, and coming directly forwards into the vitreous humour, in 

 which it was lost, after being distinctly seen for -5V tns of an inch of its course. 

 Its appearance is accurately delineated in fig. 8. 



This tube is not so distinctly seen in the eye immediately on the animal's death, 

 as some hours after; and is much more obvious in some eyes than in others. As 

 the coats of the tube must be nearly the same in all eyes, this difference probably 

 arises from its contents not always having the same degree of transparency. When 

 the eye has been kept 24 hours after the animal's death, there is an appearance of 

 a zone of a circular form, a shade darker than the rest of the eye, in which the 

 optic nerve is included: when this zone, which is nearly -3-V tns of an inch in diame- 

 ter, is attentively examined, the tube I have described is exactly in the centre of it. 

 The tube seems to be confined by the vitreous humour, while that humour is entire, 

 and only to move along with the central part of it; and in some instances, when 

 the vitreous humour is divided, the tube falls down. Its attachment at the retina 

 appears stronger than its lateral connection with the vitreous humour; for when I 

 coagulated the vitreous humour in spirits, and separated it from the retina, I found 

 the tube was left with the retina, but on being touched was easily torn. 



In the sheep's eye there is a similar tube, in exactly the same situation, respect- 

 ing the optic nerve, but much shorter, and much less easily detected. It does not 

 appear to be more than -^th °f an mcn in length, before it is lost in the vitreous 

 humour. After having seen the tube distinctly in 2 different eyes, and having had 

 a drawing made of it, I looked for it in several others, without finding it: but, 

 examining an eye from which the crystalline lens had not been removed, only an 

 aperture made into the vitreous humour, by removing a portion of the ciliary pro- 

 cesses along with the iris, the tube was distinctly seen. The weight of the lens 

 probably pulled forward the vitreous humour, and kept the short tube erect, in its 

 natural situation. In the sheep there is no appearance of a zone surrounding 

 the tube. 



These facts, though few in number, are sufficient to prove, that this orifice is 

 not peculiar to the retina of the human eye; and that its situation in man and in 

 the monkey is the same: in them, it is placed at some distance from the optic 

 nerve; but in some other animals, its situation is close to that nerve, and it puts on 

 the appearance of a tube, instead of an orifice. There is one circumstance which 

 is curious, and which it will require further information on this subject to explain ; 

 the yellow zone, found in the human eye and that of the monkey, is not met with 

 in any other animal which I have examined. Having stated the facts, and also the 

 opinions of other anatomists, that have come to my knowledge, as well as my own 

 . observations, on this orifice in the retina of the human eye, discovered by Mr. 

 Soemmering, and having added to these, several new facts respecting it in other 



