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INTRODUCTION TO NEUROLOGY 



but when the rods and cones are once actuated, they may trans- 

 mit their nervous impulses across synapses in the external 

 molecular layer to neurons of the second order whose cell bodies 

 lie in the internal granular layer. The neurons of the internal 

 granular layer are of diverse sorts, some of them spreading the 

 nervous impulse laterally (probably for summation effects in 

 weak illumination), but most of them conducting radially and 

 effecting synaptic connection with the dendrites of the "ganglion 

 cells of the retina." The latter are neurons of the third order 

 whose axons form the larger part of the fibers of the so-called 



Fig. 100. Diagram of the relations of the retina and the so-called optic 

 nerve to the other parts of the brain. 



optic nerve, which is really not a peripheral nerve at all, but a 

 true cerebral tract. 



The fibers of the "optic nerve," having reached the ventral 

 surface of the brain, enter the optic chiasma, where part of them 

 cross to the opposite side of the brain, while others enter the 

 "optic tract" of the same side. From the chiasma a big tract of 

 crossed and uncrossed optic fibers passes upward and backward 

 across the surface of the thalamus, where they divide into two 

 groups. Some terminate in the pulvinar and lateral geniculate 

 body which form the postero-dorsal part of the thalamus (Figs. 

 45, 76, 77) ; others pass these structures to end in the roof of the 

 superior colliculus of the midbrain, i. e., in the optic tectum. 



