664 DEVELOPMENT AND DISAPPEARANCE OF ELECTROTONUS. 



the (tetanic) irritation emanates from the (now separated) anode. If the current 

 is an ascending one the same operation fails to cause disappearance of the tetanus. 

 Pfliiger and v. Bezold found further evidence in favor of the view that the 

 closing contraction is due to irritation at the kathode and the opening con- 

 traction from the anode in the fact that they observed with the descending 

 current the closing contraction take place earlier after the moment of closure 

 and the opening contraction later after the moment of opening in the muscle; 

 and conversely, with the ascending current the closing contraction later, the 

 opening contraction earlier. The difference in time observed corresponds to the 

 time required for the propagation of the stimulus through the intrapolar segment. 

 If a large portion of the intrapolar segment of the nerve of a frog-preparation 

 be made inirritable by application of ammonia, only the electrode directed toward 

 the muscle will have a stimulating influence, therefore with a descending current 

 closure and with an ascending current opening. 



The law of stimulation is applicable to all kinds of nerves. 



The Law of Contraction. The contractions occurring on closing and opening 

 the circuit exhibit differences in accordance with the direction and the strength of 

 the current. 



Exceedingly feeble currents cause, in accordance with the third of the fore- 

 going propositions, and whether descending or ascending, only closing contraction. 

 The disappearance of anelectrotonus is such a feeble stimulus that the nerve 

 does not react. 



Currents of moderate strength, whether ascending or descending, cause con- 

 traction both on closing and on opening. 



Exceedingly strong descending currents cause contraction only on closing. 

 Contraction on opening is wanting because in the state of electrotonus with ex- 

 ceedingly strong currents almost the entire intrapolar segment has become 

 incapable of conduction. Ascending currents cause contraction only on opening 

 for the same reason. The rnuscle remains in contraction (closing tetanus) during 

 the period of closure with currents of a definite strength. 



Polar effects may be observed also in connection with rapid variations brought 

 about by the induced current. These cause irritation to a certain degree only at the 

 kathode in their development. At the anode the irritation is feebler on opening 

 and the irritability of the nerve is diminished in this situation. From this point 

 of view the phenomena of so-called hypermaximal contractions and deficiency are 

 to be explained. If the nerve of a frog-preparation is stimulated by a descending 

 current, which is gradually increased, the contractions at first increase in extent 

 with increase in the intensity of the irritation. On further increase, however, the 

 extent of the contractions no longer increases. If, now, the increase be continued 

 still further the height of contraction again increases hypermaximal contraction. 

 It is only with this last intensity of action that the anodal opening stimulation 

 manifests itself, and this is added to the kathodal closing stimulation, which at 

 first is alone effective. If in an analogous manner the irritation be applied by 

 means of an ascending current, the contractions will be observed to increase 

 at first with increase in the strength of the current; then with further increase 

 the contractions become smaller and they may for a time be entirely wanting 

 deficiency. This omission is explained by the action of anelectrotonus in rendering 

 conduction difficult. If the increase be made still greater, the contractions appear 

 again and they become still greater hypermaximal contractions. This last phe- 

 nomenon is explicable by the effects of anodal opening stimulation. 



The nerve in process of dying, with change in irritability according to the 

 law of Ritter-Valli, also exhibits a modified contraction-law. In the stage of 

 increased irritability, feeble currents in both directions cause only closing con- 

 traction. In the succeeding stage of commencing diminution in irritability, feeble 

 currents in both directions give rise to contraction on closing and on opening. 

 Finally, in the stage of greatly diminished irritability the descending current 

 causes contraction only on closing; the ascending, contraction only on opening. 



As the various stages of irritability advance through the nerves centrifugally, 

 the different stages may often be observed simultaneously in different segments 

 of the nerve. According to Valentin, A. Fick, Cl. Bernard, Schiff, and others, 

 the living, wholly intact nerve exhibits only closing contractions with the current 

 in either direction, but also opening contractions with currents of considerable 

 strength. 



Eckhard observed in living rabbits, with currents of moderate strength, passing 



