90 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



tivity is depressed. The stimulus on breaking takes place at 

 the anode and the impulse encounters no resistance in its passage 

 to the muscle. With the descending current the cathode lies next 

 to the muscle and the making or cathodal stimulus of course causes 

 a contraction. On breaking, however, the impulse that is started 

 from the anode is blocked by the depressed irritability in the 

 cathodal region, which, as has been said, comes on promptly with 

 strong currents and persists for a time after the current is broken. 



The Opening and the Closing Tetanus. While the du Bois-Reymond 

 law stated above expresses the facts as usually observed upon a nerve-muscle 

 preparation, there are a number of observations which indicate that the 

 excitation at the anode and the cathode during the passage of a current 

 may give rise to a series of stimuli instead of a single stimulus. Thus with 

 sensory nerves it is well known that the stimulation, as judged by the 

 sensations aroused, continues while the current is passing instead of being 

 limited to the moment of making or of breaking of the current. In this 

 respect, as in stimulation by high temperatures, the sensory fibers differ 

 apparently from the motor. When a galvanic current is passed through the 

 uinar nerve at the elbow sensations are felt during the entire time of passage 

 of the current. But in an ordinary nerve-muscle preparation it is also fre- 

 quently observed that at the moment of opening the current a tetanic con- 

 traction, persisting for some time, is obtained instead of a single twitch. This 

 phenomenon is known as the opening tetanus or Hitter's tetanus, and Pfluger 

 has shown that the continuous excitation proceeds from the anode, since 

 in the case of a descending current division of the nerve in the intrapolar 

 region brings the muscle to rest. In the same way it frequently happens 

 that upon closing the current through a nerve the muscle, instead of giving a 

 twitch, goes into a persistent tetanic contraction. The tetanus in this case 

 is designated as the closing or Pfl deer's tetanus. Both of these phenomena 

 are observed, especially, when the irritability of the nerve is for any reason 

 greater than normal. It should be added that the opening and the closing 

 tetanus may be observed also in a muscle when the galvanic current is passed 

 through it. 



Stimulation of the Nerves in Man. For therapeutic as well 

 as diagnostic and experimental purposes it often becomes desirable 

 to stimulate the nerves, particularly the motor nerves, in man. 

 We may use for this purpose either the induced (faradic, alternat- 

 ing) current or the direct battery current (galvanic or continuous 

 current) . In such cases the electrodes cannot be applied, of course, 

 directly to the nerve; it becomes necessary to stimulate through 

 the skin, and the so-called unipolar method is employed. The 

 unipolar method consists in placing one electrode, the active or 

 stimulating electrode, over the nerve at the point which it is desired 

 to stimulate, while the other electrode, the inactive or indifferent 

 electrode, is applied to the skin at some more or less remote part, 

 usually at the back of the neck. The indifferent electrode is made 

 large enough to cover several square centimeters pf the skin, and one 

 may conceive the threads of current as passing from it into the 

 moist tissues of the body, and thence to the active electrode. As 

 the threads of current condense to this latter electrode they pass 

 through the motor nerve which lies under it, and if sufficiently in- 



