HERMANN VON HELMHOLTZ 



many animals, as, for example, in the pigeon. Thus 

 a pigeon, in a sense, sees an object on its right side 

 with its left brain, and vice versa, and as the eyeballs 

 in the bird are directed to the sides, it probably sees 

 now with one eye, now with the other, and if objects 

 are immediately in front or immediately behind its 

 head, they probably form indistinct images, the bird 

 does not see distinctly, and it therefore rotates its head 

 with a well-known pert-like or quizzical action, so as 

 to bring one eye to bear on the object. In man, how- 

 ever, and in all the higher mammals, the decussation 

 is not complete, and the arrangement is probably 

 most complicated in man himself, so to meet the 

 circumstances of his erect position with eyes in front, 

 looking straight out upon the world. The fibres 

 from the nasal side of each retina cross in the com- 

 missure, while those from the temporal sides keep 

 to their corresponding sides. Thus, in the eyes of 

 a reader of this page, the nerves from the tem- 

 poral side of the right eye keep on the same side 

 in their course to the brain, but those from the 

 nasal side cross over ; on the other hand, the 

 nerves from the temporal side of the left eye 

 also keep on the same side, and those from the 

 nasal side cross in the commissure. The right 

 brain is thus related to the temporal side of the 

 right eye and to the nasal side of the left, and 

 the left brain is related to the temporal side of the 

 left brain and to the nasal side of the right. In 



