OPTIC AND VISUAL-MOTOR SYSTEMS 227 



VISUAL FUNCTIONS 



Most of our experimental knowledge of visual functions of am- 

 phibians has been derived from Amblystoma, Triturus, and frogs. 

 In these animals there is no visual cortex, and in urodeles there is no 

 specific optic projection tract to the cerebral hemisphere. Here the 

 intrinsic features of the stem portion of the visual apparatus can be 

 studied, simplified by absence of the cortical connections; and yet the 

 complications of internal structure as just summarized present baf- 

 fling physiological problems. Triturus has more efficient eyes than 

 Amblystoma, and in frogs this differentiation is much further ad- 

 vanced, so that the Anura offer the most promising approach for 

 experimental analysis, and for this we need more information about 

 the histological structure of the anuran brain than is now available. 



The electrical activity of the optic tectum of the frog has been 

 investigated by Beritoff ('43, pp. 215-40) in co-operation with 

 Tzkipuridze, who present oscillographic records of its spontaneous 

 activity and of the effects of various kinds of stimuli. A distinction is 

 drawn between the quick oscillations due to excitation of the gan- 

 glion cells of the retina and slow oscillations ascribed to activity of 

 the neuropil. Similar studies with the aid of microelectrodes may 

 profitably be made of the properties of the layers of the tectum and 

 other central stations of visual activity. 



Attention has been called ('41, p. 521) to evidence that the retina 

 of Triturus is more highly, differentiated histologically than that of 

 Amblystoma. That it is functionally superior is confirmed by the ex- 

 periments of Stone and Ellison ('45), who exchanged eyes of adult A. 

 punctatum and T. viridescens. The Triturus grafts degenerated, but 

 the Amblystoma eyes on Triturus hosts followed a course of recovery 

 as they do in homoplastic transplantations, retinal degeneration 

 being followed by complete regeneration. Visual acuity appeared to 

 be higher than in normal Amblystoma but lower than in normal 

 Triturus. 



In the feeding reactions of larval Amblystoma, the location and 

 seizure of prey can be successfully done, in the absence of eyes, with 

 the aid of the lateral-line organs (Scharrer, '32; Detwiler, '45). The 

 lemniscus systems may discharge lateral-line impulses into the tec- 

 tum, in company with those of other sensory systems; the mesence- 

 phalic V system may participate in the reaction; and the habenulo- 

 tectal tract may transmit an olfactory influence. In addition to these, 

 there is a large thalamo-tectal connection and a small strio-tectal 



