THE ELECTROMOTIVE PHENOMENA 319 



move the box so that light shall fall on the 

 retina, 



The demarcation current will undergo a nega- 

 tive variation. 



Shut off the light by replacing the box. 



There will now be a positive variation. 



Currents of action have also been demonstrated 

 in the central nervous system. Gotch and 

 Horsley find that when the spinal cord of the 

 monkey is severed, and non-polarizable electrodes 

 are applied to the longitudinal surface and the 

 cross-section, a negative variation of the current 

 of injury appears whenever the cortex of the 

 cerebrum is stimulated in the neighborhood of 

 the fissure of Rolando, the "motor " region. A 

 considerable degree of localization in the cord is 

 possible. It may be shown that the negative 

 variation from the motor region of the cortex 

 descends the cord chiefly in the crossed pyramidal 

 tract, a collection of white fibres in the lateral 

 column of the cord near the gray matter. It is 

 known from pathological evidence that the nerve 

 impulse from the motor cortical cells passes 

 through these fibres, and the demonstration of 

 their negative variation justifies the hope that 

 this method may be useful in determining the 

 course of other nerve fibres in the brain and 

 cord. 



