VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 329 



Professor Reil's mode of dissecting the eye, to show the aperture and plait, is 

 exactly similar to that mentioned in Mr. Maunoir's letter. It will appear, from 

 the account of this orifice in the retina, which precedes these observations of Pro- 

 fessor Reil, that the plait so particularly mentioned, is an artificial appearance, 

 which takes place in the dissection of the eye, and arises from the circumstance of 

 the vitreous humour adhering more firmly to the edge of this orifice, than to any 

 other part of the retina; so that the smallest motion of the vitreous humour, in 

 consequence of dividing it, or removing the choroid coat, produces a plait, by 

 pulling forwards this portion of the retina. What is said of the colour of the 

 yellow spot, and of the difference of opinion, whether it is a hole or a transparent 

 portion of the retina, I shall consider more fully in another part of this paper. 



After having ascertained the appearance of this aperture in the human eye, and 

 found what appeared the best mode of seeing it, I determined to investigate this 

 subject in the eyes of other animals. The monkey was the first animal which I 

 procured for observation; being led, from previous knowledge in comparative 

 anatomy, to believe that the structure of its eye must bear a very close resemblance 

 to that of the human subject. The eye was examined immediately after the death 

 of the animal, and was prepared in the same way that I have already described the 

 human eye to have been for this purpose; so that the concave surface of the retina 

 appeared in its most natural state, and the vitreous humour, being entire, kept it 

 expanded, and free from rugae. On the first view, nothing was to be seen but 

 one dark surface, surrounding the entrance of the optic nerve. Two hours after 

 death, the retina became sufficiently opaque to be distinguished, and immediately 

 after, the orifice was visible, appearing to be an extremely small circular aperture, 

 without any margin; but in 4- an hour more, the zone had formed, which, when 

 very accurately examined in a bright light, had an appearance of four rays, at right 

 angles, as expressed in pi. 5, fig. 7- Its situation, respecting the optic nerve, 

 was precisely the same as in the human eye. As I considered this to be a fact of 

 some importance, since it proved the aperture in the retina to be a part of the 

 structure of the eye, generally, and not a peculiarity in the human eye, I re- 

 quested Sir Jos. Banks, Sir Chas. Blagden, and Dr. Baillie, to examine it: to all 

 of them it appeared very distinct. After having shown it to those gentlemen, and 

 having an accurate drawing made of it, I preserved that portion of the eye in 

 spirits; where the aperture in the retina can still be distinctly seen, but the radiated 

 appearance is lost. 



In the eye of a bullock, prepared in the same manner, I looked in vain for a 

 similar appearance: if it existed, and bore any proportion to the size of the eye-ball, 

 as it appears to do in the human eye and that of the monkey, it must have been 

 very visible. The concave surface of the retina was examined in different lights, 

 under a variety of circumstances, and by magnifying glasses of different powers, 

 but still no aperture could be discovered. I was however very much struck, while 

 looking at the optic nerve, to see something in the vitreous humour, in conse- 

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