42 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the current may excite other parts than those which it is intended to excite 

 and false conclusions may be reached. 



In case currents of high potential are employed, another source of error 

 may arise through electrostatic charging of distant parts. 



Spread of Electrostatic Charges. — If the primary coil of an induction 

 apparatus be connected with a battery by the closure of a key in the primary 

 circuit, the sudden flow of current through the coil is accompanied by a 

 transient change in the stress of the magnetic field about the coil. This change 

 in the magnetic field induces an alteration in the electrical condition of the 

 wire of the secondary coil of the apparatus, and the terminals of this coil 

 undergo a rapid change of electrical potential, the one becoming positive, the 

 other negative. If two electrodes be connected with the binding posts of the 

 secondary coil, they become the terminals of the coil and are given, one a 

 positive, the other a negative charge. The same thing happens when the key 

 in the primary circuit is opened. In both cases the change of potential is only 

 momentary in its duration. The effect of opening the primary circuit is con- 

 siderably stronger than that of closing the circuit, for reasons stated on page 33. 



If the two electrodes are connected by a conducting material, an electric 

 current will flow from one to the other at the instant the change of potential 

 takes place. If the electrodes be connected by the nerve of a nerve-muscle 

 preparation, an electrical current will flow through the nerve ; the nerve will 

 be excited, a nerve-impulse will be developed and be transmitted along the 

 nerve to the muscle and cause it to contract. It not infrequently happens, 

 if the current entering the primary coil is strong and a large electromotive 

 force is developed in the secondary coil, that the exciting effect of the sudden 

 electrical change is not confined to the part of the nerve directly connecting 

 the electrodes, but spreads to distant parts of the nerve, and even to the 

 muscle. This is shown by the fact that the muscle will contract even after 

 a moist ligature, tied tightly about the nerve, has broken the continuity of 

 its protoplasm and so prevented the nerve impulse from reaching the muscle. 

 In such a case the contraction of the muscle is due to an irritation of the 

 nerve beyond the point to which the ligature was applied or to the direct 

 excitation of the muscle itself. 1 



If it is found that the muscle will contract after the nerve has been 

 crushed by the ligature, it will also be found that it will contract in case one 

 electrode be removed from the nerve, so that it remains connected with only 

 one pole of the induction apparatus. To understand this, we must look upon 

 the muscle as the terminal of the pole of the secondary coil with which it is 

 in connection. When the potential of the poles of the secondary coils is 

 suddenly changed, the change of potential spreads through all conducting 

 bodies connected with these poles, and in the case in question it passes, by 

 way of the wire, electrode, and nerve, to the muscle. In short, the muscle, 

 like any conductor, is charged up, and in the process of charging there is a 

 flow of current which excites the nerve and muscle. 



1 Du Bois-Eeymond : Untersuchungen iiber thierische Electricitat, Bd. i. S. 423. 



