THE PERCEPTION OF SPACE 699 



the left hemisphere. Although the projection of the two corresponding 

 retinal areas upon the same cortical field is not essential for the fusion 

 of their several sensory impressions, such a confluence of sensory- 

 conductors is necessary, as was pointed out by Mott (1905) and 

 Sherrington (1906), if they are to have access to a common efferent 

 (motor) path which both must use if a coordinated mechanism is to 

 result. 



The complete decussation of the optic nerve fibres at the chiasma 

 in Vertebrates below Mammalia and their partial decussation in 

 Mammalia is thus associated with the fact that the latter is the only 

 class of Vertebrates wherein the ocular movements are coordinated. 

 Moreover, the latter is the only class of Vertebrates wherein the ocular 

 motor nuclei in the mid-brain, particularly those of nerves III and VI, 

 are intimately related with a system of crossed association fibres 

 (Kappers, 1920). The anatomical association of the visual fibres is thus 

 an evolutionary adaptation correlated with motor rather than sensory 

 events, and marks a distinct stage in the progress of the development 

 of binocular vision into a highly integrated mechanism of ever-increasing 

 exactitude. Without complete motor coordination the continually 

 shifting system of local signs of direction characteristic of animals 

 with uncoordinated eyes could not have been replaced by a functionally 

 established system of corresponding points and accurately fixed local 

 signs of direction, nor would it have been possible to introduce 

 additional clues to the judgment of distances such as physiological 

 diplopia. With such coordination, community of sensation becomes 

 reinforced by community of action. Significantly, the appearance of 

 such coordination in Mammals coincides with the fact that in these, 

 for the first time, the visual processes are transferred from the tectum 

 and the mid-brain to the cortical level ^ ; only in Mammals, therefore, 

 is such coordination possible. 



Kappers (1920) accounted for the partial decussation of the visual fibres 

 by the theory of neurobiotaxis, a hypothesis by which he has endeavoured to 

 explain the complicated migration of nerve centres and nerve tracts in phylo- 

 genetic history, and the seemingly peculiar location and relation in which this 

 has resulted in the higher animals. In its essentials the theory postulates that 

 the migration and final arrangement of neural elements are determined by an 

 association of function, the determining force being physico-chemical. The 

 intimate nature of such a force or the manner of its action is, of course, highly 

 speculative — and admittedly so ; but such a conception, correlating structure 

 and function, Is essentially rational in its biological implications, and clears up 

 many difficult points in the anatomy of the central nervous system of the higher 

 animals, in its comparative anatomy, and in its embryology. 



Those parts of the central nervous system which are associated with the 

 photostatic fimctions of vision provide several peculiarly apt illustrations of this 

 theory. The most outstanding, perhaps, is the position of the oculomotor 



1 p. 543. 



