38 EVOLUTION OF THE HUMAN EYE 



the left halves of the two retinae superimposed on the left 

 half on the occipital cortex, and those from the right halves 

 of the two retinas on the right half of the occipital cortex, 

 but that the impressions received from each macula are 

 similarly superimposed on each half of the brain (Fig. 7). 

 Clinically this view helps us not only in explaining the 

 "macula escape" in cases of bilateral hemianopsia, but 

 also affords an explanation of cases of central hemianopic 

 defects without any peripheral contraction of the fields, 

 an affection which, if right-sided, has the characteristic 

 symptom of the patient finding a difficulty in reading while 

 still retaining good central acuity of vision. 



This semi-decussation of the nerve fibers from each macula 

 is also, I suggest, of interest in connection with the causation 

 of certain cases of congenital concomitant strabismus. As 

 we all know, there are cases in which children have per- 

 fect acuity of vision in each eye, with little or no refractive 

 error, who never at any time make an attempt to use the 

 two eyes together, nor can they by any means be induced 

 to do so. A very simple explanation of such cases might 

 be found in a congenital defect in the course of the macular 

 nerve fibers, whereby complete decussation, instead of semi- 

 decussation, had taken place. With complete decussation 

 the impression received by the visual cortex from each 

 macula- would be unilateral instead of bilateral, as with 

 semi-decussation. The acuteness of perception would not 

 be affected by the complete decussation, only the stereoscopic 

 effect. The anatomical evidence in proof of this hypothesis 

 would be exceedingly difficult to obtain. Clinical evidence 

 might, however, be forthcoming should a patient having 

 such a congenital concomitant strabismus subsequently 

 develop a homonymous bilateral hemianopsia, in which 

 case, if this hypothesis be true, we should expect to find the 



