;86 



THE EYE AND VISION. 



[CH. LV. 



centres controlling these phenomena, and by the arrangement of 

 the nerve-fibres in the optic nerves. The crossing of the nerve 

 fibres at the optic chiasma is incomplete, and the preceding 

 diagram (fig. 597) gives a simple idea of the way the fibres go. 



It will be seen that it is only the fibres from the inner portions 

 of the retinae that cross ; and that those represented by con- 

 tinuous lines from the right side of the two retinse ultimately 

 reach the right hemisphere, and those represented by interrupted 



HI Nerve 



Fig. 598. Relations of nerve cells and fibres of visual apparatus. (After Schiifer.) 



lines from the left side of the two retinae ultimately reach the 

 left hemisphere. The two halves of the retinae are not, however, 

 separated by a hard-and-fast line from one another ; this is repre- 

 sented by the two halves being depicted as slightly overlapping, 

 and this comes to the same thing as saying that the central 

 region of each retina is represented in each hemisphere. The 

 fibres (represented by finely dotted lines in the diagram) connect- 

 ing the two retinae and the two hemispheres are problematical. 



The part of the hemisphere concerned in vision is the occipital 

 lobe, and the reader should turn back to our previous considera- 



