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 

 retinse 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 retinse 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 crus cerebri and 

 finally divides in the roof of the mid- and 

 fore-brain into three branches, which end in 

 the grey matter of the anterior corpora quad- 

 rigemina and in the external geniculate body 

 and the pulvinar of the optic thalamus. 

 Running 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 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 retina?. 

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



4' 15 



Via. 203. Diagram to show con- 

 nections of optic tracts. (After 

 SHERRTNGTON.) 



L, left, and K, right retina ; OB, 

 optic decussation (chiasma) ; OpT, 

 optic tract; NC, nucleus caudatus ; 

 LN, lenticular nucleus ; Th, optic 

 thalamus ; G. external geniculate 

 body ; AQ, anterior corpus quadri- 

 geminum ; P, pulvinar ; OpR, optic 

 radiations running to OC, the occi- 

 pital cortex ; Illn , nucleus of third 

 nerve in floor of Sylvian aqueduct ; 

 TV, fourth ventricle. 



