AQ 



III 



SECTION XIV 

 VISUAL REFLEXES 



FOREMOST among the afferent impulses determining the reactions 



of higher animals are those arising in the eyes. Each retina, or rather 



the two retinsB acting together as a single 



organ, can be regarded as a sensory 



surface, each point of which corresponds 



to a point, or series of points, lying in 



a given direction outside the body. 



Each optic nerve contains about half 



a million nerve fibres, i.e. as many as 



enter the cord by the posterior roots 



from the whole of the body. The two 



optic nerves coming from the retinae 



meet together in the floor of the fore- 



brain and form the chiasma. At the 



chiasma a decussation of fibres takes 



place, which, in animals such as the 



rabbit, with no fusion of the fields of 



the two eyes, is practically complete. 



In man only those fibres which arise in 



the mesial half of each retina cross the 



mesial plane ; these, together with the 



uncrossed fibres from the temporal half 



of the other retina, form the optic tract 



of the opposite side (Fig. 203). The ., 



Optic tract passes backwards across the optic decussation (chiasma) ; OpT, 



eras cerebri and finally divides into three $^ t ^^^ 



branches, in the roof of the mid- and thalamus ; G, external geniculate 

 f i -i i i 1 1 body ; AQ, anterior corpus quadri- 



f ore-brain, which end in the grey matter gera inum7 P, pulvmar; OpR, optic 



of the anterior corpora quadrigemina and radiations running to OC, the occi- 



., i i . i j i J.T. pital cortex ; Illn, nucleus ol third 



in the external geniculate body and the nerve in floor of Sylvian aqueduct 

 pulvinar of the optic thalamus. Running IV, fourth ventricle. 

 in the optic tract are also fibres which 



are simply commissural ; these form the mesial root of the optic 

 tract. They cross in the optic chiasma and serve to connect the 

 two internal gcniculate bodies. In addition to the afferent fibres 



459 



203. Diagram to show con- 

 tracts * (After 



