86 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 

 ulnar 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 Pfliiger 

 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 Pfliiger's tetanus. Both of these phenomena 

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

 greater than normal. It seems probable, therefore, to many observers that 

 the excitation at the cathode persists in reality during the passage of the 

 current even in motor fibers, although ordinarily the excitation makes itself 

 felt upon the muscles only at the moment of closure ; the excitations during 

 the passage of the current being either too weak to affect the muscle or the 

 condition of the nerve being such as to prevent their conduction to the muscle. 

 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 of the skin, and one 

 may conceive the threads of current 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 intense will 

 stimulate the nerve. The arrangement is represented in the accom- 

 panying schema (Fig. 33), showing the disposition of the electrodes 



