VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 331 



animals, I shall draw some general conclusions from the whole, with a view to 

 show that the conjectures which have been made, respecting its use, are probably 

 erroneous. I shall afterwards point out several reasons for considering it as the 

 orifice of a lymphatic vessel intended to carry off the vitiated parts of the vitreous 

 humour and crystalline lens. 



In the human subject, as no examination can be made for some considerable 

 time after death, it is impossible to ascertain what is the real state of this orifice in 

 the living eye, and what changes take place in it after death ; we only learn, that 

 the tinge of yellow surrounding the orifice is very slight, when the eye is examined 

 recently, and that the next day it becomes much deeper. These points appear to 

 be satisfactorily cleared up, by the examination that was made of the monkey's 

 eye, as it was begun before the parts had lost the appearance belonging to them as 

 living parts. In that state the retina was transparent, and no orifice could be seen; 

 so that the orifice is rendered visible by remaining transparent, while the surround- 

 ing retina becomes opaque. This appears to decide the dispute between Messrs. 

 Soemmering and Buzzi ; for if this part does not undergo the change peculiar to 

 the retina, we must consider the retina as wanting there. After the orifice is thus 

 rendered visible, the yellow tinge is wanting, and does not take place for several 

 hours, and even then is fainter than it becomes afterwards; which appears to be 

 sufficient evidence, that this tinge is the effect of some change after death, and 

 cannot therefore have any effect on vision. 



The orifice has been supposed to account for a small object becoming invisible, 

 when placed at a certain distance from the eye, and brought opposite a particular 

 part of the retina. This however cannot be the case, as its situation in the retina 

 does not correspond with the part opposed to the object, when rendered invisible. 

 The orifice itself is probably too small to produce any defect in vision, as the trunks 

 of the blood-vessels which ramify on the retina cover a larger space than this orifice 

 for a considerable extent, without obstructing the sight of any part of the object. 



While my observations were confined to the human eye, I was led to consider 

 this orifice as a lymphatic vessel, passing from the vitreous humour through the 

 retina, but could bring no absolute proof of its being so. This opinion was 

 strengthened by finding, that in the monkey, the orifice was only rendered visible 

 when the retina became opaque; and it has since been corroborated, by a distinct 

 tube being met with in the eyes of sheep and bullocks. That a change must be 

 constantly taking place in the crystalline and vitreous humours, to preserve to them 

 the necessary degree of transparency, can hardly be doubted ; and that the absor- 

 bent vessels which perform that office should have one common trunk, which follows 

 the course of the artery and vein, perfectly agrees with what takes place in other 

 parts of the body. In the human eye, and that of the monkey, the artery is in 

 the centre of the optic nerve; but that would have been too circuitous a course for 

 the lymphatic vessel to follow, and by going through the retina, at some distance 

 from the nerve, it can pass out of the orbit with the blood-vessels that go through 



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