134 ELECTROTONUS. [BOOK i. 



will flow in the direction of the arrow. Similarly the needle of G will 

 by its deflection indicate the existence of a current flowing from g to g 

 through the galvanometer, and from g to g through the nerve, in the 

 direction of the arrow. 



At the instant that the polarizing current is thrown into the nerve 

 at pp' t the currents a,tgg', hti will undergo a "negative variation," that is, 

 the nerve at each point will exhibit a "current of action" correspond- 

 ing to the nervous impulse, which, at the making of the polarizing 

 current, passes in both directions along the nerve, and may cause a 

 contraction in the attached muscle. The current of action is, as we 

 have seen, of extremely short duration, it is over and gone in a small 

 fraction of a second. It therefore must not be confounded with a 

 permanent effect which, in the case we are dealing with, is observed in 

 both galvanometers. This effect, which is dependent on the direction 

 of the polarizing current, is as follows : Supposing that the polarizing 

 current is flowing in the direction of the arrow in the figure, that is, 

 passes in the nerve from the positive electrode or anode/? to the negative 

 electrode or kathode p', it is found that the current through the 

 galvanometer G is increased, while that through H is diminished. The 

 polarizing current has caused the appearance in the nerve outside the 

 electrodes of a current, having the same direction as itself, called the 

 ' electrotonic ' current ; and this electrotonic current adds to, or takes 

 away from, the natural nerve-current or " current of rest " according as 

 it is flowing in the same direction as that or in an opposite direction. 



The strength of the electrotonic current is dependent on the strength 

 of the polarizing current, and on the length of the intrapolar region 

 which is exposed to the polarizing current. When a strong polarizing 

 current is used, the electromotive force of the electrotonic current may 

 be much greater than that of the natural nerve-current. 



The strength of the electrotonic current varies with the irritability, 

 or vital condition of the nerve, being greater with the more irritable 

 nerve ; and a dead nerve will not manifest electrotonic currents. More- 

 over, the propagation of the current is stopped by a ligature, or by 

 crushing the nerve. 



We may speak of the conditions which give rise to this electrotonic 

 current as a physical electrotonus analogous to that physiological electro- 

 tonus which is made known by variations in irritability. The physical 

 electrotonic current is probably due to the escape of the polarizing 

 current along the nerve under the peculiar conditions of the living 

 nerve ; but we must not attempt to enter here into this difficult subject 

 or into the allied question as to the exact connection between the 

 physical and the physiological electrotonus, though there can be little 

 doubt that the latter is dependent on the former. 



77. These variations of irritability at the kathode and anode 

 respectively thus brought about by the action of the constant 

 current are interesting theoretically, because we may trace a con- 

 nection between them and the nervous impulse which is the result 

 of the making or breaking of a constant current. 



For we have evidence that a nervous impulse is generated 

 when a portion of the nerve passes suddenly from a normal 



