228 THE BRAIN OF THE TIGER SALAMANDER 



tract. But there is no experimental evidence about the specific func- 

 tions of any of these connections. 



The tectum and thalamus are connected by fibers passing in both 

 directions (figs. 11, 12). These are precursors of the mammalian 

 brachia of the superior and inferior colliculus. It is evident that the 

 tectum and the dorsal thalamus may act as an integrated unit in 

 regulating the major activities of the body in adjustment to external 

 situations. With the emergence of the cerebral cortex in higher ani- 

 mals, this pattern is radically changed, and for the understanding of 

 these changes it is essential that the pre-existing organization be ade- 

 quately known. The history of the transformation of this generalized 

 amphibian structure to those of reptiles and mammals has been 

 sketched by Huber and Crosby ('43). 



Evidence has been published that in some vertebrates the several 

 quadrants of the retina have local representation in the optic tectum. 

 In a series of vertebrates, including several species of amphibians, 

 Stroer ('39, '39a, '40) described continuous fascicles of fibers from the 

 retinal quadrants to the tectum, where they end locally in an ar- 

 rangement which projects the retinal quadrants upon the tectum in 

 reversed orientation. I have made diligent search for evidence of such 

 an arrangement in Necturus and in young and adult Amblystoma 

 without success ('41, '41a, '42). In Amblystoma thick and thin fibers 

 from the different retinal quadrants are inextricably intertwined in 

 the optic nerve. At the chiasma the thick .fibers are segregated from 

 the thin fibers, and in the tectum the terminals of both kinds of 

 fibers mingle and seem to be distributed nearly uniformly to all 

 parts. There is physiological evidence that Amblystoma can visually 

 localize objects in the field of view; and, by analogy with other ani- 

 mals, the presumption is strong that retinal loci are projected upon 

 the tectum in more or less definitely circumscribed areas. 



In Triturus and Amblystoma good localization of objects in the 

 visual field is restored after transplantation of the eyes (Stone, '44; 

 Stone and Zaur, '40; Stone and Ellison, '45), and it is preserved after 

 rotation of the eyeball through 180°, though in the latter case motor 

 reactions to objects in the visual field are reversed from the normal 

 (Sperry, '43). Similar physiological results follow operations which 

 eliminate the chiasma and connect the retina with the tectum of the 

 same side, and also after transplanting an eye to the opposite side 

 of the head (Sperry, '45). These observations have been repeated 

 upon seven species of anurans, all of which have much more highly 



