PHYSIOLOGICAL INTERPRETATIONS 87 



system also illustrates the principle just stated. In most vertebrates 

 the eyes are the dominant organs concerned with the orientation of 

 the body and its members in space. The visual apparatus within the 

 brain, accordingly, exhibits the most precise localization of function, 

 and the refinement of this localization increases progressively in the 

 phyletic series. 



In Amblystoma the fibers of the optic tracts are widely spread in 

 the brain stem, in marked contrast with those of other sensory sys- 

 tems, which tend to converge into a single receptive field. There is 

 no evidence that this dispersal of fibers of retinal origin to the tec- 

 tum, pretectal nucleus, thalamus, hypothalamus, and cerebral pe- 

 duncle is correlated with any specificity of visual function. The ex- 

 planation of this central spread of retinal fibers is to be sought on the 

 motor side of the arc, that is, it is determined primarily by the nature 

 of the response to be evoked. 



In the Amphibia all these visual areas are centers of correlation, 

 for in all of them optic terminals are mingled with those of other sys- 

 tems. Within this class, however, as we pass from generalized uro- 

 deles to specialized anurans, there is a conspicuous trend toward 

 segregation of some terminals of the optic nerve in the tectum and 

 lateral geniculate body of the thalamus. This trend culminates in 

 mammals and is correlated with the differentiation of the visual area 

 of the cerebral cortex, until in primates, as pointed out by Clark 

 ('43), the retinal-geniculate-cortical pathway provides a very pre- 

 cise point-to-point projection of the visual field upon the cerebral 

 cortex, and "there is no possibility that these impulses can be dis- 

 turbed and modified 'en route' by other, unrelated, types of nervous 



impulse In other words, the cerebral cortex receives retinal 



impulses in a remarkably pure and unadulterated form." 



The highly specialized optic tecta of some lower vertebrates ex- 

 hibit two types of specific localized structure, which differ in form 

 and physiological significance. There is, first, an arrangement of 

 sensory terminals spread superficially in mosaic pattern. This pro- 

 vides for point-to-point projection of retinal loci upon the tectum and 

 perhaps for other forms of sensory localization. In the second place, 

 there is a pattern of lamination at different depths from the surface. 

 These laminae differ somewhat in their sensory connections, and the 

 sensory influence is stronger in the moi-e superficial members of the 

 series. The arrangement of the deeper layers seems to be determined 

 primarily by the directions taken by their efferent fibers. The mosaic 

 pattern is primarily in terms of sensory analysis, the lamination 



