CM. LV.] THE OPTIC NERVES. 7^5 



of the diagram is shown the way in which the brain combines the 

 images in the two retinae, one as it were overlapping so as uo 

 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 



Left Retina Right Retina 



Right 

 Hemisphere Hemisphere 



Fig. 596. The Horopter, when the Fig. 597. Course of fibres at 



eyes are convergent. optic chiasma. 



near object, the horopter is a circle (fig. 596) 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 0'). All other points in 

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

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

 a very simple mathematical problem to prove that OA = 0'A', 

 and OB = 0'B'. 



This, however, applies to man only, or to animals with both 

 eyes in front of the head ; in those animals in which the eyes 

 are lateral in position, and the visual lines diverge, the problem 

 of bilateral vision is a very different one. 



Nervous Paths in the Optic Nerves. 



The correspondence of the two retina? and of the movements 

 of the eyeballs is produced by a close connection of the nervous 

 K.P. 3 E 



