146 



THE PHYSIOLOGY OF NERVE 



excitation. The writing levers (W), attached to the two ends of the muscle, are 

 adjusted in the same ordinate, so that any difference in the onset of the contractions 

 in the two halves will be indicated in the record. On making the current by closing 

 the key (k), the contraction invariably begins at that end of the muscle which is 

 connected with the cathode (C), while on breaking the current the end joined with 

 the anode (A) is activated first. The polarity of the muscle is then changed by 

 reversing the bridge of the Pohl commutator (F) interposed in the circuit, so that 

 the previously cathodic end now becomes anodic. Although reversed as far as 

 the muscle is concerned, the results will be identical with the preceding. This 

 experiment may be modified in the following manner. It is a well-known fact that 



a much more lasting character may be im- 

 parted to the contractions by the use of a 

 strong galvanic current. The one obtained on 

 making the current is designated as Wendt's 

 tetanus and the one on opening, as Ritter's 

 tetanus. Engelmann has proved that these 

 tetanic contractions remain confined to that 

 end of the muscle in which they originate, 

 namely, the making tetanus to the cathodic 

 and the opening tetanus to the anodic end. 



The phenomenon of electrotonus may also 

 be reproduced with the help of the simple 

 core-model described in one of the preceding 

 paragraphs, but naturally, the conditions here 

 met with are purely physical in their nature 

 and are not complicated by physiological 

 changes, as they are in living nerve. Thus, it 

 has beennoted that an electrolytic dissociation 

 takes place between the metal core and the 

 surrounding solution whenever a current is 

 passed through it. The cathodic ions are 

 made to move toward the anode and the 

 anodic toward the cathode until true electro- 

 tonic currents have been produced. 



Pfliiger's Law of Contraction. — In 



order to show that the passage of a 

 galvanic current gives rise to a cathodic 

 area of excitation and an anodic area 



Fig. 86. — Method of Testing 



THE ElECTROTONIC CONDITION OF 



Nerve. 



K, key for making and break- 

 ing of constant current; P, pole 

 changer for reversing current so of depression, these regions Hiav be i 

 that either pole may be made ,• i . j j. i. • i« • j^ ^ 'J^ 



anodic or cathodic; s, point of stimulated at brief intervals either 

 stimulation of nerve by means of mechanically or by means of single in- 

 induction shocks; w, writing lever duction shocks (Fig. 86). In the latter 



attached to muscle. ., i , \ / n\ , i -.i 



case, the electrodes (o), connected with 

 the secondary coil of an inductorium, are placed in the immediate 

 vicinity of either the positive or negative non-polarizable electrode. 

 By using a strength of induction which, when brought to bear upon the 

 cathodic region, just barely produces a contraction of the muscle, it 

 can easily be shown that this same stimulus applied to the anodic 

 region, fails to incite a reaction. But even if the same minimal 

 stimulus is employed for both regions, a comparison of the amplitude 

 of the contractions then resulting will show immediately that the one 

 obtained by stimulating at the cathode, is the larger of the two. In 



