UNIPOLAR EXCITATION. 463 



Finally, the vital electromotive changes present during the functional 

 activity of electrical organs, voluntary muscle, cardiac muscle, and even nerve 

 itself, may under appropriate conditions he employed as nerve stimuli. 1 



(6) Unipolar excitation. — The employment of electrical currents for 

 stimulation is, unless special precautions are observed, liable to lead to 

 erroneous conclusions, through difficulty in localising the seat of their 

 application. This is especially the case when the electromotive source 

 is one of high potential, and one fruitful source of error is the existence 

 of what is termed unipolar excitation. This phenomenon is readily 

 demonstrated when induction currents are employed. If the sciatic 

 nerve of the frog is connected with one terminal only of the secondary 

 coil, the opening or closing of the primary circuit may still evoke a 

 muscular response. The most careful insulation of the preparation is 

 ineffectual to eliminate the exciting effect, but it disappears if the 

 terminal connected with the nerve is in addition adequately con- 

 nected with the earth by means of water-pipes, or the like. If the 

 intensity of current employed is just inadequate to evoke the 

 response, it becomes adequate when either the unconnected terminal of 

 the coil, or any part of the insulated preparation, is touched with the 

 finger: it is even possible to cause an excitation by approaching the 

 nerve with the moist finger. 



The cause of the phenomenon is the unequal surface distribution of 

 electrical potential through the whole conducting mass, including the 

 preparation. 2 With high potential, such as that of the induced current, 

 any alteration in surface distribution is necessarily accompanied by the 

 passage of considerable currents during the period of alteration; the 

 rapidity with which this takes place is very considerable, owing to the 

 quick development of the electromotive force, and thus increases the 

 exciting value. It is possible to obtain such variations in surface 

 distribution by suspending an insulated nerve muscle preparation in 

 air, in the neighbourhood of Leyden jars connected with the poles of a 

 suitable Motional machine ; every time the distribution is altered by a 

 discharge between the poles, a response occurs, owing to a similar 

 variation of distribution in the preparation, and such response is 

 augmented if the preparation is touched by the finger. 



The use of electromotive changes of high potential is thus always liable 

 to produce unipolar effects ; it is for this reason that in the case of induced 

 currents, especially break induced currents, the presence of a closing and open- 

 ing key directly interposed in the secondary exciting circuit is inadmissible, 

 and a short circuiting key, with effective metal contacts, must be employed. 



Unipolar effects are a fruitful source of error, as changes will be constantly 

 produced by the existence of currents which, being inadequate to evoke 

 muscular effects, are disregarded ; in all investigations upon the electromotive 

 phenomena of nerve, the possibility of their presence must be always con- 

 sidered, and as far as possible guarded against. 3 The employment of unipolar 

 excitation offers some special advantages, and in the hands of Charpentier and 

 others has yielded results which are obtained with difficulty, if at all, by other 

 modes of stimulation. 4 



1 Hering, "Biological Memoirs," Oxford, 1889 ; Sitzungsh. d. k. Akad. d. Wissensch.. 

 Wien, 1882, Brl. lxxxv. 



2 E. du Bois-Reymond, "Untersucli. ii. thier. Elektricitiit." 



3 Hering, SUzungsb. d. k. Akad. d. Wissensch., Wien, 1884, Bd. lxxxix. Abth. 3. 



4 Charpentier, Compt. rend. Soc. de biol., Paris, 1894 ; Arch, dcphysinl. norm. et path., 

 Paris, 1894, etc. 



