228 THE NERVOUS SYSTEM 



arising in the cortex and terminating in the pulvinar, lateral geniculate body, 

 and superior colliculus of the corpora quadrigemina. 



The significance of the partial decussation of the nerves is made clear by 

 Figs. 162 and 163. The properties of the refracting media of the eyes are such 

 that images of objects to the left of the axis of vision are produced on the nasal 

 side of the left eye and the temporal side of the right eye. And, due to the man- 

 ner of decussation of the optic nerve-fibers, impulses from both these sources 

 reach the visual area of the right cortex. In the same way the visual cortex 

 of the left side receives impressions from objects to the right of the axis of vision. 

 That is to say, the sensory representation of the outer world in the cerebral 

 cortex is contralateral in the case of sight just as it is in the case of cutaneous 



Fig. 163. Diagram to show why a destruction of one optic tract causes blindness in both eyes for 

 the opposite lateral half of the field of vision. 



and auditory sensations. Furthermore, it will be evident that, while destruc- 

 tion of one optic nerve causes total blindness in the corresponding eye, destruc- 

 tion of one optic tract, its thalamic connections, their optic radiations, or the 

 visual cortex in which these radiations terminate, will produce blindness in both 

 eyes for the opposite lateral half of the field of vision. This condition is known 

 as hemianopsia, and is produced by a lesion in the optic pathway anywhere be- 

 hind the chiasma. 



