EVOLUTION OF BINOCULAR VISION 329 



been surgically scrambled — ^pp. 311-2) we then have a situation superior 

 to that in, say, the chameleon, each of whose eyes has to locate the prey 

 insect by itself before the cerebral navigator can work out the position 

 of the insect by a process of triangulation. 



Partial decussation of the optic nerves accomplishes just this desirable 

 tying-up of both retinae to both the left-brain and the right-brain centers 

 of eye-muscle control. In the thalamus, not far from the groups of nuclei 

 which operate the eye muscles, there are way-stations on the sensory 

 pathway from the retina to the cerebral cortex. These way-stations, the 

 lateral geniculate bodies, are connected by way of the superior coUicuU 

 with the nuclei of the eye-muscle nerves. Here, then, is the real terminus 

 of the optic nerve fibers so far as concerns any importance of the fact 

 that they come into each side of the thalamus from both retinae instead 

 of from only one. The fact that in the higher vertebrates ^/-retinal im- 

 pulses continue on up to each half of the cerebral cortex then becomes 

 altogether meaningless; for in whatever patchwork fashion the two op- 

 tical images finally arrive at the cortex, the two (left and right) cortical 

 image-patterns are due to be fused into one pattern anyway. The whole 

 aim and goal of partial decussation has already been attained down in 

 the thalamus and the tegmentum, where what one eye is seeing is enabled 

 to control the motor impulses to both sets of eye muscles. 



Partial decussation is thus explained, not as the indispensable basis of 

 binocular single vision, but as a logical eventual consequence of binocu- 

 larity. Its value is not in the immediate field of conscious sensory phe- 

 nomena at all, but in the realm of motor activity where it serves to facili- 

 tate the motor cooperation of the two eyes. Partial decussation has never 

 arisen in the owls or the frontal-eyed deep-sea fishes, perhaps not because 

 (or not only because) these are not mammals, but because their eyes are 

 motionless. 



The evolution of their motor conjugation has made the mammals 

 completely dependent upon it for singleness of binocular vision and for 

 accurate space-perception, for along with it there arose the phenomenon 

 of corresponding points. This dependence is at once spot-lighted when 

 anything goes wrong with an eye muscle or its nerve, and a strabismus 

 or squint develops — one eye turning out or in so far that diplopia occurs. 

 We can perhaps best understand the relation of corresponding points to 

 eye-movement conjugation, and understand how the lower vertebrates 

 get along without both, if we consider the phenomenon of the substitute 

 macula. 



