330 ADAPTATIONS TO SPACE AND MOTION 



Invariably in a strabismus patient the fovea of the inturned eye loses 

 its directional sign; but occasionally a patch of the nasal retinal periph- 

 ery, which is now aimed into space along a line parallel to the other eye's 

 visual axis, takes on the quality of a corresponding spot paired with the 

 fovea of the good eye. The previous diplopia slowly fades away and the 

 patient becomes capable of fusing the image on the fovea of the normal 

 eye with that on the 'substitute fovea' of the squinting eye. If now the 

 squinting eye is straightened by an operation, its fovea will regain its old 

 community of direction with the fovea of the other eye, and temporarily 

 there will be a monocular diplopia in the operated eye — until the latter's 

 substitute macula has had time to 'fade' and regain its original notion of 

 direction. 



These processes, under favorable conditions, may require weeks or 

 months. We can describe the essentials of what has happened, by saying 

 that a spot on the retina of the squinting eye has taken on a new local 

 sign of direction because the eye has taken a new position in the head, 

 with the result that the same objects, in the same places in space, are now 

 seen in those same places, as before. When we have said all this, we have 

 really also described what happens in the lower vertebrates with total 

 decussation, when they perform their independent eye movements. The 

 only difference is that the alteration of local directional signs is contin- 

 uous and instantaneous as the animal turns the eyes about, while in man 

 the local signs are so firmly fastened to particular retinal points that 

 changing them is an extremely slow process and is seldom possible at all! 

 The mammals indeed lost something when they developed the partial 

 chiasma for the sake of conjugating their eye movements. 



In conclusion, then : the vertebrates which have much of a binocular 

 field have always had singleness of objects in that field, and perceive 

 them as 'solidly' as their inter-pupillary distances allow. The need of a 

 permanent coordination of eye movements for rapid and precise estim- 

 ation of distance was finally met in the mammals by the device of partial 

 decussation in the optic chiasma, putting the oculomotor apparatus in 

 control of the pair of images and making it responsible for the main- 

 tenance of fusion. Conjugation being attained, a system of fixed local 

 retinal signs of direction could now develop, with a consequent improve- 

 ment of the precision of localization through the appearance of a new 

 cue to distance — the physiological diplopia (and haziness) of objects 

 which are off the horopter, and which only disappears when convergence 

 and accommodation have hit their mark precisely. But, the animal now 



