276 PROTOPLASMIC ACTION AND NERVOUS ACTION 



the factor arousing the local excitation at each successive 

 region in a transmitting element (nerve, etc.) — attains 

 its full intensity not instantaneously but in a rising 

 curve of more or less gradual slope, varying from tissue 

 to tissue, and subsides in a similar manner. Stimulation 

 by currents of varying intensity is thus the typical 

 condition prevailing in the organism, and to which, 

 therefore, especial attention must be directed. The 

 general empirical rule governing the action of such 

 currents is as follows: 



3. In order to stimulate, a current rising continuously 



and uniformly from zero to full intensity must change 



its intensity at a certain minimal rate which is charac- 



Ai 

 teristic for the tissue; i.e., —= const, (assuming 



temperature and other conditions normal). Hence we 

 find that a slowly mcreasing current may fail to stimulate, 

 while one rising to the same intensity at a more rapid 

 rate stimulates. The same rule applies to stimulation 

 by the decrease of a current already flowing through 

 a tissue. The rate of change, in either direction, must 

 exceed a minimal value which is specific for the tissue. 

 A time-factor enters, closely related to that already 

 referred to above (under 2) as ''chronaxie." 



4. The stimulating action of the current is charac- 

 teristically polar; i.e., the current produces its primary 

 physiological effects chiefly at its regions of entrance and 

 exit, and the effects at the two regions are typically 

 opposite or antagonistic. Typically, when the current 

 is made, it initiates excitation at the cathode (i.e., where 

 the positive stream of the stimulating circuit passes 

 from the tissue to the applied electrode), and inhibits 



