GRADATION OF EXCITABILITY 165 



When the stimuhis was apphed from above or from 

 below the pulvinus, careful observation of the recording 

 lever showed a curious erectile twitch preceding the normal 



Fig. 91. Diagrammatic representation of the different layers 

 in a longitudinal section of pulvinus. 



Cortex of upper half extends from a to vascular bundle [f ; 

 b, uppermost layer of lower half ; c, hypothetical most 

 sensitive layer ; d, lowest layer of cortex of lower half. 

 Vertical continuous arrow represents application of light 

 on upper, dotted arrow on lower, side. 



response. This was especially the case when the tonic con- 

 dition of the specimen was not exceptionally high, so that 

 the rate of percolation of excitation was relatively slow. 

 A satisfactory record of the preliminary twitch, due to 

 positive impulse, was obtained by the employment of a 

 higher magnification. 



