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INTRODUCTION TO NEUROLOGY 



being given off to terminate in the unspecialized correlation 

 centers of the dorsal part of the thalamus. But in all forms with 

 a differentiated cerebral cortex these thalamic optic connections 

 assume greater importance, a special region in the dorsal part 

 of the thalamus being set apart for their use. Thus arose the 

 lateral geniculate body, and in higher mammals this is supple- 

 mented by the pulvinar. These centers are, in the strict sense 

 of the word, cortical dependencies, for they attain to only very 



vfc I 



Fig. 102. Section through the parietal eye of a lizard (Anguis fragilis) : 

 ct, Connective-tissue cells around nerve; gc, ganglion cells; I, lens;-n, nerve- 

 fibers; pc, pigment cells; pn, parietal nerve from the parietal eye to the 

 brain; r, retinal cells; vb, vitreous body. (After Nowikoff.) 



insignificant proportions in forms with rudimentary cerebral 

 cortex, but increase in proportion to the elaboration of the visual 

 cortex. 



The early steps in the evolution of the eyes of vertebrates are imperfectly 

 understood. In structure and mode of function the vertebrate eyes are 

 unlike those of any of the invertebrate animals. The experiments of Parker 

 and others have shown that the skin of many aquatic vertebrates among the 

 fishes and amphibians is sensitive to light, and it has been supposed that the 

 vertebrate retina was differentiated from such cutaneous photoreceptors. 

 But it seems more probable (Parker, 1908) that the vertebrate organs of 

 vision were developed from the first within the central nervous system. 



Some of the fishes and reptiles possess, in addition to lateral eyes of 

 typical form, a median eye, the parietal or pineal eye (Fig. 102), which is 



