CH. LVL] 



NERVOUS PATHS IN THE OPTIC NERVES 



807 



The accompanying diagrams will assist us in understanding what 

 is meant by corresponding or identical points of the two retinae. 



If K and L (fig. 596) represent the right and left retime 

 respectively, and 0' the two yellow spots are identical ; so are A 

 and A', both being the same distance above and 0'. But the 

 corresponding point to B on the inner side of in the right retina, 

 is B', a point to the same distance on the outer side of (X in the left 

 retina ; similarly C and C' are identical. The two blind spots X and 

 X' are not identical. 



Fig. 597 shows the same thing in rather a different way ; A and 

 B represent a horizontal section through the two retinae ; the points 

 a a', b b', and c c', being identical. In the lower part of the diagram 



FIG. 597. Diagram to show the correspond- 

 ing parts of both retinae. 



O A 



FIG. 598. The Horopter, when the 

 eyes are convergent. 



is shown the way in which the brain combines the images in the 

 two retinae, one overlapping so as to coincide with the other. 



The Horopter is the name given to the surface in the outer world 

 which contains all the points which fall on the identical points of 

 the retinae. The shape of the horopter will vary with the position 

 of the eyeballs. In the primary position, and in the first variety of 

 the secondary position, the visual lines are parallel; hence the 

 horopter will be a plane at infinity, or at a great distance. 



In the other variety of the secondary position, and in tertiary 

 positions in which the visual lines converge, as when looking at a 

 near object, the horopter is a circle (fig. 598) which passes through the 

 nodal points of the two eyes, and through the fixed point (I) in the 

 outer world at which the eyes are looking, and which will con- 

 sequently fall on the two yellow spots (0 and (X). All other points 

 in this circle (II, III) will fall on identical points of the retinae. 

 The image of II will fall on A and A' ; of III on B and B' ; it is a 



