320 



ADAPTATIONS TO SPACE AND MOTION 



50% in the higher primates and a low minimum in lateral-eyed forms; 

 but even the rabbits have some uncrossed fibers. This relationship is the 

 'law of Newton-Miiller-Gudden', and holds good only for the mammals. 

 Outside of that class, there is no case of a partial decussation of any 

 degree whatever. 



Supposed Value of Partial Decussation — A few have thought that 

 partial decussation arose as a device for preserving, in animals with 

 frontal or partly frontal eyes, the original status in which the left brain 

 saw everything that was to the right of the animal and the right brain 



Fig. 118 — Illustrating Ramon y Cajal's ex- 

 planation of the decussation of the optic 

 nerves, a, situation which would obtain if 

 the nerves did not decussate: the two halves 

 of the visual field are transposed, b, the 

 decussation of the nerves makes the sub- 

 jective visual field a proper panorama. 



Fig. 119 — Illustrating Ovio's correction of 

 Ramon y Cajal's vievi: since the whole 

 extent of any object in the binocular field is 

 seen by each eye, and since the separate 

 mental images are due to be fused inter- 

 hemispherically anyway, it makes no differ- 

 ence whether the nerves decussate or not, 

 a, without decussation, b, with it. 



(In Figs. 118-121 J. the left- and right-eyed aspects of the visual field are respec- 

 tively indicated by the solid and dotted portions of the visual object [arrow]). 



kept watch on the left — the situation which obtains in a lamprey, for 

 example, where there is total decussation and no binocular field at all. 

 But this naive view presupposes that the ancient invention of total decus- 

 sation was somehow of vital importance in the first place ; and, still worse, 

 it rides rough-shod over the fact that Gudden's law is inoperable in lower 

 groups despite the presence in them of species with even total frontality 

 (some deep-sea fishes, owls, and — dynamically — chameleons). 



The great majority of physiological opticists have instead seen in par- 

 tial decussation the essential basis of fusion and stereopsis. The argu- 

 ment is that since there are no median end-stations in the brain, fusion 

 must occur on each side and can only do so if each half of the brain 



