in the Return of the Eye. 343 
The orifice itself is probably too small to produce any defect 
in vision, as the trunks of the blood-vessels which ramify 
upon 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 w'as strength- 
ened by finding, that in the monkey, the orifice was only ren- 
dered 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 crys^ 
talline and vitreous humours, to preserve to them the necessary 
degree of transparency, can hardly be doubted ; and that the 
absorbent 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 cir- 
cuitous a course for the lymphatic vessel to follow, and, by go- 
ing through the retina, at some distance from the nerve, it can 
pass out of the orbit with the blood-vessels that go through the 
foramen lacerum orbitale inferius. In the bullock and sheep, 
there is a plexus of vessels surrounding the optic nerve, and the 
tube dips down, close by the optic nerve, probably to accom- 
pany them. 
From the observations made by Miciiaelis, of the yellow 
spot not being visible in foetuses, or in infants under a year old, 
