ELECTRICAL PHENOMENA OF NERVE 191 



through the galvanometer back to the starting point. Points 

 equidistant from the centre on the cross-section and from the 

 equator on the longitudinal surface are at the same potential. 

 A current obtained by mutilation of the tissue is known as a 

 current of injury, of rest, or of demarcation. In a cat's nerve it 

 has been found to equal 0.01 and in an ape's nerve 0.005 of a 

 Daniell cell. Dead tissue gives no current. 



It has been found than when living tissue is stimulated, the 

 activity accompanied by katabolic changes sweeps over it in 

 the form of a wave. As the latter, in muscle or nerve, passes 

 by the electrodes it brings about differences of potential which 

 are indicated by a galvanometer. These differences of potential 

 give rise to currents of action. When a current of action is 

 superimposed upon a current of rest, the needle of the galvan- 

 ometer, having been deflected to a certain extent by the latter, 

 is made to move back toward the zero point, giving rise to a 

 negative variation. When the nerve of one (A) of two nerve- 

 muscle preparations is laid lengthwise over the muscle of the 

 other preparation (B), and the nerve of B is stimulated with 

 an interrupted current, both muscles are thrown into tetanus. 

 That this phenomenon is not due to a spread of the exciting 

 current through the preparations is shown by ligating the nerve 

 B between the electrodes and the muscle, when all contractions 

 cease. As a matter of fact, the muscle fibers of B give rise to 

 currents of action that are circuited through the fibers of nerve 

 resting on its surface. The nerve fibers of A are thus stimulated 

 and cause the muscle to contract. This phenomenon is known as 

 secondary tetanus. As the wave of a nerve impulse sweeps by 

 the electrodes, it changes the potential successively of one and 

 then of the other, so that the needle of the galvanometer is 

 deflected at one instant in a positive direction, and in the next 

 in a negative direction. Currents of action are therefore 

 diphasic. Changes of irritability due to the passage of a con- 

 stant current have been alluded to under electro tonic changes. 

 There are to be observed at the same time variations in the 

 electrical currents of the nerve itself, i. e., variations in the cur- 

 rents of rest. The constant current causes in the nerve outside 

 of the electrodes the appearance of another current that has 

 the same direction as itself, and is called the electrotonic current. 

 The electrotonic current adds to or takes away from the cur- 



