808 THE EYE AND VISION [CH. LVI. 



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 binocular 

 vision is a very different one (see also p. 713). 



Nervous Paths in the Optic Nerves. 



The correspondence of the two retinas 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. 599) 

 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 retinas that cross; and that those 



Left Retina Right Retina represented by continuous lines from the 

 right side of the two retinas ultimately reach 

 the right hemisphere, and those represented 

 by interrupted lines from the left side of the 

 two retinas ultimately reach the left hemi- 

 sphere. 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 depicted as slightly 

 overlapping, and this comes to the same 

 W '"Right thing as saying that the central region of 



Hemisphere Hemisphere % t - -i i 



each retina is represented in each hemisphere. 



FIG. 599. Course of fibres at _., ^ , , . , r , . 



optic chiasma. The part of the hemisphere concerned in 



vision is the occipital lobe, and the reader 



should turn back to our previous consideration of this subject in 

 connection with cerebral localisation, the phenomena of hemian- 

 opsia (p. 689) and the conjugate deviation of head and eyes 

 (pp. 689, 703). 



Fig. 600, though diagrammatic, will assist the reader in more 

 fully comprehending the paths of visual impulses, and the central 

 connections of the nerves and nerve-centres concerned in the process. 

 The fibres to the external geniculate body end there by arborising 

 around its cells, and a fresh relay of fibres from these cells passes 

 in the posterior part of the internal capsule to the cortex of the 

 occipital lobe. Those to the anterior corpus quadrigeminum are 

 continued on by a fresh relay to the nuclei of the nerves concerned 

 in eye-movements (represented by the oculo-motor nucleus in the 

 diagram); the axons of the cortical cells pass to the tegmentum, 



