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 



retinae acting together as a single organ, can 



be regarded as a sensory surface, every 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. 



4t 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 cms cerebri and 



in-,..-,.,,, i T_ . ,1 ,. FIG. 203. Diagram to show con- 



finally divides into three branches, in the root nections of optic tracts. (After 



of the mid- and fore-brain, which end in the SHERRINGTON.) 



grey matter of the anterior corpora quadri- b^o^ra^tfen^f^^^^T! 



gemina and in the external geniculate body optic tract ; NO, nucleus caudatus ; 



and the pulvinar of the optic thalaniUS. thalamus CU <3 external geniculate 



Running in the Optic tract are also fibres body; AQ, anterior corpus quadri- 



! i -, i ,1 p geminum ; P, pulvinar ; OpR, optic 



which are simply commissural ; these form Radiations running to OC, the occi- 



the mesial root of the optic tract. They cross pital cortex ; Illn, nucleus of third 



. , , . n , , nerve in floor of Sylvian aqueduct ; 



in the optic chiasma and serve to connect the iy, fourth ventricle. 

 two internal geniculate bodies. In addition 



to the afferent fibres from the retina to the brain the optic tract contains a 

 certain number of efferent fibres which pass out and end in the retinse. 

 It is evident from these connections that whereas section of one optic 



406 



