CH. LVIIL] 



THE OPTIC NERVES 



853 



A - ' 



FIG. 540. The Horopter, when the 

 eyes are convergent. 



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

 will be a plane at an infinite 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. 540) 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 consequently fall on the two yellow 

 spots (0 and 0'). 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 simple mathematical problem to 

 prove that OA = 0'A', and OB = 0'B'. 



In those animals in which the eyes 

 are lateral in position, and the visual 

 lines diverge, the problem of binocular 

 vision is a very different one (see also 

 p. 736). 



Nervous Paths in the Optic Nerves. 



The correspondence of the two retinae and of the movements of 

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

 controlling these phenomena, and by the arrangement of the nerve- 

 fibres in the optic nerves. The crossing of the nerve-fibres at the 

 optic chiasma is incomplete, and the next 

 diagram (fig. 541) gives a simple idea of the 

 way the fibres go. 



It will be seen that it is only the fibres 

 from the inner portions of the retinae that 

 cross; and that those represented by con- 

 tinuous lines from the right side of the two 

 retinae ultimately reach the right hemisphere, 

 and those represented by interrupted lines 

 from the left side of the two retinae ultimately 

 reach the left hemisphere. The two halves 

 of the retinae are not, however, separated by 

 a hard-and-fast line from one another; this 

 is represented by the two halves being de- 

 picted as slightly overlapping, and this comes 

 to the same thing as saying that the central region of each retina is 

 represented in each hemisphere. 



The part of the hemisphere concerned in vision is the occipital 

 lobe, and the reader should turn back to our previous consideration 



Left Retina Right Retina 



Hemisphere 



Right 

 Hemisphere 



FIG. 541. Course of fibres at 

 optic chiasma. 



