GENERAL PHYSIOLOGY OF NERVE-TISSUE. 127 



appearance of catelcctrotonus is more effective as a stimulus than the 

 disappearance of anelectrotonus. For these reasons the term polar 

 stimulation is generally employed in discussing the make and break 

 effects of the galvanic current. /The law of contraction may then be 

 explained as follows^ Very feeble currents, either ascending or de- 

 scending, produce contraction only upon tHe closure of tlie circuit, the 

 sucTden increase of the excitability in the catelectrotonic area being 

 alone sufficient to generate an impulse. The contraction which 

 follows the closing of the weak ascending current depends upon the 

 fact that the decrease of excitability and conductivity at the anode is 

 insufficient to interfere with the conduction of the cathodal stimulus. 



, Medium currents, either ascending or descending, produce contrac- 

 tion both on closing and opening the circuit. The appearance of 

 caTelectrotonus and the disappearance of anelectrotonus are both 

 sufficiently powerful to generate an impulse without, however, seri- 

 ously impairing the conductivity of the nerve. 



X^Very strong currents produce contraction only upon the opening 

 of the ascending and closure of the descending currents^flf upon the 

 passage of the excitability in the former from the marked anelectro- 

 tonic decrease to the normal condition, and in the latter from the nor- 

 mal to that of catelectrotonic increase. The absence of contraction 

 upon the closure of the ascending current is dependent upon the 

 blocking of the cathodal stimulus by the decrease of the excitability 

 and conductivity at the anode. With the opening of the descending 

 current the disappearance of anelectrotonus should also be followed 

 by contraction, which would indeed be the case if the stimulus so 

 generated was not blocked by the decrease of the conductivity at the 

 cathode. 



The order in which the contractions occur may be tabulated as 

 follows : 



With Ascending Current. With Descending Current. 



Weak, i. K. C. C.* K. C. C. 



Medium, 2. K. C. C. A. O. C.f K. C. C. A. O. C. 



Strong, 3. __ A. O. C. K. C. C. A. O. C.(?) 



Polar Stimulation of Human Nerves. The preceding state- 

 ments as to changes in the excitability caused by the passage of a 

 constant current, as well as to the law of contraction, are based en- 

 tirely on experiments made with the isolated nerve of the frog. It 

 is probable, however, that the same phenomena would have been 

 observed had the nerve of a mammal been used and its excitability 

 been maintained. 



If the electrodes connected with the wires of a sufficiently strong 

 galvanic battery be applied to the skin over the course of a superficially 



* K. C. C., cathodal closing contraction, f A. O. C., anodal opening contraction. 



