506 NER VE. 



the exclusion of the centripolar electrode (A), without interfering with the 

 resistance or current intensity ; in the figure the opening of K l and 7i 3 , and 

 the closure of K^ must produce the familiar bipolar arrangement of contacts 

 B, C. 



FIG. 264. 



Danilewsky found that all stages of Pflliger's law were demonstrable 

 with the tripolar as with the ordinary bipolar arrangement, but, with 

 the tripolar arrangement, excitation both at closure and at opening was 

 effectually produced by an intensity of current which was inadequate 

 with bipolar contacts. This difference was obtained whether the 

 direction of the currents was through the myopolar and centripolar 

 regions of nerve to the middle point, or the reverse. It thus appears 

 that the doubling of the intrapolar region favours excitation by 

 electrical currents, even when this doubling is one which involves the 

 presence of two symmetrically altered regions meeting at a middle point. 



Both the length of nerve submitted to the action of the current and 

 the number of poles are thus clearly shown to be important factors in 

 determining the efficiency of excitation by electrical currents. The 

 importance of the duplication of poles is further demonstrated by 

 experiments in which alternating induced currents are used for excita- 

 tion of a motor nerve. With the tripolar arrangement a muscular 

 response can be evoked with induced currents which are quite in- 

 adequate to cause the response with the ordinary bipolar contacts. 



The effect of the duration of the current upon polar excitation has 

 been incidentally mentioned several times. The rapid development of 

 catelectrotonus, and the slower development of anelectrotonus, causes 

 currents of moderate strength, but of short duration, to excite only on 

 closure, i.e. at the cathode. This is the case with condenser discharges, 

 break induced currents, and short galvanic currents. In the case of the 

 latter, a limit can be reached even for this cathodic excitation of medul- 

 lated nerves, the minimum exciting duration at 15 C. being '0015 sec. ; x 

 it appears to be longer in nerves cooled to C., amounting then to "02 

 sec., 2 and is also longer in nerves the excitability of which has been 

 lowered by excision, etc. 3 With non-medullated nerves (Anodon) the 

 limit and consequent failure is reached with still longer duration. 4 



The experimental proof of the main fact, that induced currents, 

 unless intense, excite at the cathode only, is readily effected by the 

 use of local alterations of temperature in the manner previously in- 

 dicated, for these currents excite better when the seat of excitation is 



1 Fick, "Untersuch. u. elektr. Nervenerregung," 1864. 



2 J. Konig, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1870, Bd. Ixii. Abth. 2. 



3 Neumann, Arch. f. Anat., Physiol. u. wissensch. Med., 1864, S. 554. 



4 Biedermann, "Elektrophysiologie," 1895, S. 546. 



