Double and Inverted Images on the Retine. 485 
neral law certainly is, that all impressions made on their branches 
are felt by us to be higher or lower, as the points of the cerebro- 
spinal axis, from which they originate, and on which their sensi- 
bility depends, are truly higher or lower ; and if there be such a 
peculiarity in the insertion of the optic nerves into the cerebro- 
spinal axis, that its highest portion is inserted lowest, and its 
outermost portion inserted innermost, then the fact of impres- 
sions on the upper surface of the retina and optic nerve being 
felt by us as lower, and of impressions on their outer surface 
being felt as inner, will be reduced to the same law as regulates 
our perception of the relative position of objects of touch. 
In all the vertebrated animals, it is well known that the optic 
nerves, behind the commissure or decussation, or Tractus optici, 
cross and embrace the Crura cerebri ; and that in the Mammalia 
they are in connexion, behind or above the crura, with the bodies 
called Thalami nervorum opticorum, and Corpora quadrigemina. 
It has been disputed, even lately, whether the true origin of 
the optic nerves in the human body is in the Thalami, as was 
formerly thought, or in the Corpora quadrigemina, as maintained 
by Gat and Spurzuerm ; and I have repeatedly noticed the 
accuracy of the observation of the late Dr Gorpon, that the 
fibres of the tractus opticus are not merely expanded over the 
outer surface of the thalamus in the human body, but at various 
points plunge into its interior. Even those which thus dip in- 
wardly, however, follow the same direction as the more superfi- 
cial fibres, tending inwards and downwards towards the Corpora 
quadrigemina. ; 
In others of the mammalia, the connexion of the optic nerves 
with the thalami is much more superficial ; and in birds, reptiles, 
and fishes, it seems perfectly ascertained, that the optic lobes, in 
which the optic nerves exclusively originate, correspond to the 
Corpora quadrigemina only. 
The question of the true origin of the optic nerves, however, 
cannot be decided merely by anatomical inquiry. It is now well 
3Q2 
