Double and Inverted Images on the Retina. 483 



points on the retinae (at least in those corresponding points on 

 the retinae 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 retinae 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 retinae 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 II. 3 Q 



