PEOOF OF ELECTROTONUS IN SENSORY AND INHIBITORY NERVES. 567 



excitability ; only above and below the anode (at c c) the positive current emerges from the 

 nerve and excites cathelectrotonus at these points. Similarly, where the cathode is applied 

 ( - c c) there is increased excitability, but in higher and lower parts of the nerve, where (at a a) 

 the positive current (coming from + ) enters the nerve, the excitability is diminished (anelec- 

 trotonus) (v. Helmholtz, Erb). If we desire to stimulate in the neighbourhood of an electrode, 

 then we cannot act upon that part of the nerve whose excitability is influenced by the electrode. 

 In order, therefore, to stimulate directly the same point on which the electrode acts, it is 

 necessary to apply the stimulus at the same time by the electrode itself, e.g., either mechani- 

 cally or by conducting the stimulating current through the polarising circuit ( Waller and de 

 Watteville). 



II. Proof of Electrotomis in Sensory Nerves. Isolate the sciatic nerve of a decapitated 

 frog. When this nerve is stimulated in its course with a saturated solution of common salt, 

 reflex movements are excited in the other leg, the spinal cord being intact. These disappear as 

 soon as a constant current is applied to the nerve, provided the salt lies in the anelectrotonic 

 area {Pfliiger and Zurhelle, Hallsten). 



III. Proof of Electrotomis in Inhibitory Nerves. To show this, proceed thus : On causing 

 dyspnoea in a rabbit, the number of heart-beats is diminished, owing to the action of the 

 dyspnceic blood on the cardio-inhibitory centre in the medulla oblongata. If, after dividing 

 the vagus on one side, a constant descending current be passed through the other intact vagus, 

 the number of pulse-beats is again increased (descending extrapolar anelectrotonus). If, how- 

 ever, the current through the nerve be an ascending one, then with vjeak currents the number 



Fig. 410. 

 Scheme of the distribution of an electrical current in the nerve on galvanising the 



ulnar nerve. 



of heart-beats increases still more (ascending extrapolar cathelectrotonus). Hence, the action of 

 inhibitory nerves in electrotonus is the opposite of that in motor nerves. 



During the electrotonus of muscle, the excitability of the intrapolar portion is 

 altered. The delay in the conduction is confined to this area alone {v. Bezold) 

 compare 337, 1. 



336, ELECTROTONUS LAW OF CONTRACTION. Opening and Closing 

 Shocks. A nerve is stimulated both at the moment of the occurrence and that of 

 disappearance of electrotonus {i.e., by closing and opening the current Bitter): 

 (1) When the current is closed, the stimulation occurs only at the cathode, i.e., at 

 the moment when the electrotonus takes place. (2) When the current is opened, 

 stimulation occurs only at the anode, i.e., at the moment when the electrotonus 

 disappears. [This is Pfliiger's well-known principle " A given tract of nerve is stimu- 

 lated by the appearance of cathelectrotonus and the disappearance of anelectrotonus not, 

 however, by the disappearance of cathelectrotonus nor by the appearance of anelectro- 

 tonus." From this principle can be deduced the law of contraction.] (3) The 

 stimulation at the occurrence of cathelectrotonus is stronger than that at the dis- 

 appearance of anelectrotonus {Pfliiger). 



Bitter's Opening Tetanus. That stimulation occurs only at the anode, when the current is 

 opened, was proved by Pfliiger by means of "Ritter's opening tetanus." Ritter's tetanus con- 

 sists in this, that when a constant current is passed for a long time through a long stretch of 

 nerve, on opening the current, tetanus- lasting for a considerable time results. If the current 



