34° Mr. Home's Account of an Orifice 
Its appearance is accurately delineated in the annexed plate. 
(Fig- 4 ) 
This tube is not so distinctly seen in the eye immediately 
upon 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 pro- 
bably arises from its contents not always having the same de- 
gree 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 ^ths of an inch 
in diameter, 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 coagu- 
lated the vitreous humour in spirits, and separated it from the 
retina, I found the tube was left with the retina, but upon be- 
ing touched was easily torn. 
In the sheep's eye there is a similar tube, in exactly the same 
situation, respecting the optic nerve, but much shorter, and 
much less easily detected. It does not appear to be more than 
~^th of an inch in length, before it is lost in the vitreous hu- 
mour. After having seen the tube distinctly in two 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 
