298 THE NERVOUS SYSTEM 



Binocular perception adds a further method of judging 

 distance. In animals such as man which have parallel 

 optical axes the visual fields of the two eyes overlap to 

 a considerable extent. Rays of hght coming from any 

 point in the common field of vision stimulate corresponding 

 points on the two retinas, so that the two stimuli are fused 

 centrally to form one visual impression. For instance, an 

 object situated in front of another forms an image on the 

 temporal side of the other on the left retina and an image 

 on the nasal side on the right retina, yet these corresponding 

 images cause but one image to be formed in consciousness. 

 The images formed on the two retinae are thus not exactly 

 the same, and it is the fusion of these shghtly different 

 images in the brain which gives us stereoscopic vision. 



CENTRAL CONNECTIONS OF THE OPTIC NERVES 



The two optic nerves join together at the optic chiasma, 

 where a partial decussation takes place. The fibres from 

 the nasal half of the retinae cross over, while those 

 from the temporal half remain on the same side ; fibres 

 from each fovea being partly crossed, partly uncrossed. 

 The regrouped fibres are conveyed by the optic tracts to 

 the brain-stem, where thev terminate in three nuclei, the 

 anterior corpora quadrigemina, the Oftic thalamus and the 

 external geniculate bodies. In the anterior corpora quadri- 

 gemina some fibres arborise round nuclei of the third and 

 fourth cranial nerves; others arborise around cells which 

 pass down the brain-stem in the posterior longitudinal 

 bundle and thus bring the optic nerves into functional con- 

 nection with the other cranial and the spinal nerves. This 

 connection provides a means of co-ordination between 

 visual impressions and the muscles of the Umbs. 



From the optic thalamus and external geniculate bodies 

 there starts a fresh relay of fibres which pass through the 

 posterior hmb of the internal capsule to end in the occipital 

 lobe in the anterior part of the calcarine fissure. Destruc- 



