GENERAL PHYSIOLOGY OF NERVE-TISSUE. 109 



both conditions being attended by a rise of excitability not, however, by 

 the opposite changes. It is further assumed that the appearance of kate- 

 lectrotonus is more effective as a stimulus than the disappearance of anelec- 

 trotonus. 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 descending, produce contraction only upon the closure 

 of the circuit, the sudden increase of the excitability in the katelectrotonic 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 inferfere with the conduction of the kathodal stimulus. Medium currents, 

 either ascending or descending, produce contraction both on closing and 

 opening the circuit. The appearance of katelectrotonus and the disap- 

 pearance of anelectrotonus are both sufficiently powerful to generate an im- 

 pulse without, however, seriously impairing the conductivity of the nerve. 



Very strong currents produce contraction only upon the opening of the 

 ascending and closure of the descending currents, or upon the passage of the 

 excitability in the former from the marked anelectrotonic decrease to the 

 normal condition, and in the latter from the normal to that of katelectrotonic 

 increase. The absence of contraction upon the closure of the ascending 

 current is dependent upon the blocking of the kathodal stimulus by the 

 decrease of the excitability and conductivity at the anode. With the open- 

 ing 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 kathode in consequence of the fall of a high state of katelectrotonus 

 to the normal condition. 



The order in which the contractions occur may be tabulated as follows: 



With Ascending Current. With Descending Current. 



Weak i. K. C. C. 1 K. C. C. 



Medium 2. K. C. C. A. O. C. 2 K. C. C. A. O. C. 



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



Polar Stimulation of Human Nerves. The preceding statements as 

 to changes in the excitability caused by the passage of a constant current, 

 as well as to the law of contraction, are based entirely 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 gal- 

 vanic battery be applied to the skin over the course of a superficially lying 

 nerve, e.g., the brachial, it will be found that there occurs on the closure of 

 the circuit an increase in the excitability in the extra-polar anelectrotonic 

 region and a decrease in the excitability in the extra-polar katelectrotonic 

 region, as shown by stimulating the nerve in the extra-polar regions with 

 the induced current results which are in apparent contradiction to those 

 obtained with the isolated nerve. This want of accordance in the results 

 of the two classes of experiments arises from a failure to recognize the fact 



*K. C. C., kathodal closing contraction. a A. O. C., anodal opening contraction. 



