Double and Inverted Images on the Retine. 483 
points on the retine (at least in those corresponding points on 
the retina which can be brought to bear on the same objects), 
originate in the same points on the optic lobes. Now, in order 
that this may be effected, there must be a very peculiar arrange- 
ment of the nervous matter, both on the retinz in front, and on 
the optic lobes behind. The entrance of the optic nerve in the 
human eye being considerably on the inner side of the optic axis, 
the separation of the right and left portions of the retina cannot 
take place there ; the fibres on the inner side of the nerve, com- 
ing from the opposite lobe, must extend outwards as far as the 
central foramen, in order that all the inner half of the retina 
may be connected with the opposite optic lobe ; and, at that cen- 
tral point of the retina, these fibres, or the membrane continuous 
with them, must be overlapped by those which come from the 
lobe on their own side, and form the outer part of the optic 
nerve. Again, the fibres passing backwards from the outer por- 
tion of the right, and inner portion of the left retina, to form 
the right tractus, must be there so combined, as that those 
which come from corresponding points in the retinze may be im- 
planted at the same points in the lobes. And I am not aware 
that these peculiarities in the course of the fibres, which the 
theory seems obviously to require, have hitherto been detected 
' by any anatomist. 
It is to be observed here, however, and obviates one objec- 
tion that has been stated to this theory, that in the case of 
most quadrupeds and birds, whose eyes are generally directed 
widely asunder, it can only be a small segment of the outer part 
of the retina of either eye, which will ever be brought to bear on 
objects situated within the sphere of vision of the other eye ; and 
it is only this small portion which, if the principle of single vision 
has been correctly stated, requires to be associated at its root with 
the corresponding portion of the retina of the other eye. Ac- 
cordingly, in such animals, it is obviously more than a semi-de- 
cussation which takes place at the commissure of the optic nerves ; 
VOL. XIII. PART I. 3@ 
