SECTION XIV 

 VISUAL REFLEXES 



FOREMOST among the afferent impulses determining the reactions 



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



the two retinae 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 



... 

 ot the opposite side (Fig. 203). The 



, , , . 



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



rms rprpbri anrl finallv rh\n'rlp<5 into thrpp optictract; NC, nucleus caudatus; 



LN, lenticular nucleus ; Th, optic 

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



fore-brain, which end in the py matter J3fcSSSJ5K* 



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



, i 1-1 11 i J.T. pital cortex ; Illn. nucleus of third 



m the external geniculate body and the ^rve 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 geniculate bodies. In addition to the afferent fibres 



459 



203. Diagram to show con- 

 nections of optic tracts. (After 

 SHERRINGTON.) 



T . ,, . ^ . , , 



L, left, and R, right retina.; 



