434 



Comparative Animal Physiology 



employing the retinal action potential, optic nerve discharge, or some overt 

 organismic response, and their comparison v\'ith the spectral absorption curves 

 of known animal photolabile pigments, such as rhodopsin. Similarly, meas- 

 urements of the time course of dark adaptation by different techniques agree 

 quite well. Further, the reciprocity law of Bunsen and Roscoe has been dem- 

 onstrated by a variety of techniques to apply to photoreceptors at given re- 

 sponse levels. The conclusion that both retinal and optic nerve activity de- 

 pends on the absorption of radiant energy by a photolabile pigment seems to 

 be justified. 



30 SEC. 



10 MIN. 



w^/m^^Kf^m 



I MIN. 



2 MIN. 



40 MIN. 



5 MIN. 



I HR. 



Fig. 138. Spike potentials recorded from a single optic nerve fiber (LjwmZ«s) in 

 response to brief and constant illumination of the eye at various times in the dark (indi- 

 cated at the left of each record), after a period of light adaptation. Time in 1/5 sec. 

 The light flash is indicated by interruption of the white line. From Hartline and Mac- 

 Donald." 



What then is the relation between the retinal action potential and the 

 activity in the nervous elements of the optic pathway? This question is not 

 easily answered, although it has been raised a number of times.*'^- ^'^' "^ 

 It has been suggested, by Hartline^'^ and others, that the retinal action po- 

 tential produces local action currents which initiate activity in the nervous 

 elements of the optic pathway. This sequence of events, that is, a slow 

 potential followed by nerve discharge, is not unknown in nerve physiology. 

 Such a sequence has been demonstrated in the ganglion of the heart of Limu- 

 lus, ^^' ""^ and there is some evidence for a similar sequence of events in 

 the lower motor neurones of the vertebrate spinal cord. 



Attractive as this suggestion may be, there is no evidence that affords clear- 

 cut proof. It has been shown that the retinal action potential precedes the 



