SINGLE VISION. 395 



skin, so far as waste and restoration are concerned, can not fail to be 

 noticed. 



The effect which has thus been communicated to the vesicular layer 

 of the retina, through the intervention of Jacob's rods and T 



. interconnection 



cones, is now earned along the nervous tubules out of the of the right and 

 globe of the eye. The nerves from each eye, converging, e fc cye> 

 encounter one another at the chiasm, the triple structure of which has 

 already been described. Here it is, however, to be understood that, 

 while the proper optic tubules of the right eye go to the left brain, and 

 of the left eye to the right brain, the anterior band of commissural tu- 

 bules brings the two eyes into a special relation with one another, the 

 right side of one eye corresponding with the rignt of the other, and the 

 left with the left ; or, to put the same statement under a more simple yet 

 more instructive form, the outer side of one eye corresponds with the in- 

 ner of the other, and in this manner the two retinas become as if they 

 were virtually incased the one within the shell of the other, an arrange- 

 ment which obviously, as has been already remarked, compensates in a 

 degree for the blind spot of each eye, and, indeed, eliminates the effect of 

 all accidental irregularities, for numberless such irregularities must exist, 

 there being a necessity, for example, that blood-vessels should cross 

 through the sensitive to the conducting structures, and such blood-ves- 

 sels give rise to lines of inertness. 



From this commissural arrangement it comes to pass that each retina 

 possesses regions of symmetry with the other, and on this single and 

 singleness of vision depends ; each point of the outer portion double vision, 

 of the retina of the right eye has its point of symmetry in an inner portion 

 of the left, and when from a distant object rays fall on these symmetrical 

 points, that object will be seen single ; but if, by the pressure of the fin- 

 ger or otherwise, we compel the image to fall in one of the eyes upon 

 another, and, therefore, non-symmetrical point, the object at once becomes 

 double. It should be remarked that this exchange of symmetry concerns 

 only the lateral divisions, for the upper portion of one eye corresponds 

 with the upper portion of the other, and the lower with the lower. 



If the view which I have presented respecting the scal^e of the laby- 

 rinth of the ear be correct, that singular structure finds its Analo be _ 

 equivalent in the black pigment of the eye ; for though we tween the scalse 

 only know in an indistinct manner the physical condition of and P. 1 s ment - 

 black opacity, we may be certain that it arises from total interference of 

 rays, and such, it is presumed, is the office of the scalaa of the ear. 



Impressions made upon the retina do not disappear instantly, but grad- 

 ually fade away, and in so doing occupy a certain period of Duration of 

 time, which varies with the brightness of the original light, the impressions 

 existing condition of the eye, and the illumination to which it 



