322 ADAPTATIONS TO SPACE AND MOTION 



eye then sees only fields in which the other eye can never see anyway, 

 and the crescentic uniocular temporal retinal fields involved are totally 

 decussated. Still, we are perfectly able to observe 'out of the comers' of 

 our two eyes simultaneously. There is no alternation, no rivalry, no 

 attempt to fuse and discomfort from its failure. 



The great Spanish neurologist Ramon y Cajal believed that in forms 

 with total decussation there must be panoramic vision, the visual fields of 

 the two eyes being subjoined to complete the whole picture of space 

 (Fig. 118). Without total decussation, he argued, the two halves of the 

 whole field would be transposed in the animal's mind (Fig. 118a) and 

 vision would become worse than useless for purposes of spatial localiz- 

 ation and orientation. He thought that where there is partial decussation, 

 fibers coming from corresponding points in the two retinae ended up at 

 single points in the visual cortex, in 'isodynamic cells' which accom- 

 plished the fusion. This theory might seem reasonable enough where the 

 foveae are concerned; but all other 'corresponding points' are imperma- 

 nent and it would take an infinity of isodynamic cells to tie together 

 all possible combinations of them. The whole matter of corresponding 

 points is a psychological one, and not anatomical in any way, as the 

 phenomenon of the substitute macula (v.i.) clearly shows. 



Ovio has corrected Ramon y Cajal's idea of the panorama, which was 

 based of course on the mistaken belief that a binocular field is an excep- 

 tion rather than the rule among vertebrates. Ovio's diagrams (Figs. 119a 

 and 119b) bring out how little difference it makes to binocular vision 

 whether there is total decussation or no decussation at all. Ovio believes 

 that fusion (by superposition, not by continuity) takes place in animals 

 with total decussation, since psychic fusion is a joining of the images 

 in the two hemispheres into one phenomenally median image; but he 

 goes on (Fig. 120) to explain partial decussation as a device for making 

 the mental image larger, and 'therefore' better resolved. On this point, 

 his reasoning becomes very hard to follow. 



Ovio believes, with others, that solidity results from bringing together 

 two disparate views of the object in the same center — i.e. one side (either 

 side) of the brain — but that a 'psychic act' is still necessary to fuse them 

 into a single solid image. The psychic act of fusion does not in itself 

 create the relief, however; for even when we have only one eye open, 

 that eye is evoking activity in both cerebral hemispheres and these two 

 cerebral actions are being somehow unified, yet there is no resultant 

 idea of relief. 



