338 Mr. Home's Account of an Orifice 
plait so particularly mentioned is an artificial appearance, which 
takes place in the dissection of the eye, and arises from the cir- 
cumstance 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, whe- 
ther 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 ob- 
servation ; 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 rugce. 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 aper- 
ture, without any margin ; but, in half an hour more, the zone 
had formed, which, when very accurately examined in a bright 
