ELECTROTONUS 489 



Electric Current from the Exterior to the Cut Surface of a Nerve. 

 Before studying certain phenomena presented in nerves of which a por- 

 tion is subjected to the action of a constant galvanic current, it is impor- 

 tant to note the fact that there exists in the nerves, as in the muscles, a 

 current with a direction from the exterior to the cut surface. It has 

 been roughly estimated that the nerve-current has one-eighth to one- 

 tenth the intensity of the muscle-current. The existence of the nerve- 

 current has, so far as known, no more physiological significance than 

 the analogous fact observed in the muscular tissue. Currents exist also 

 in the skin and in mucous membranes, the direction being from the 

 outer surface, which is positive, to the inner surface, which is negative. 1 



Electrotonus, Anelectrotonus and Catelectrotonus. When a constant 

 galvanic current is passed through a portion of a freshly-prepared 

 nerve, a large part of the entire nerve is brought into a peculiar electric 

 condition. While in this state, the nerve will deflect the needle of a 

 galvanometer and its excitability is modified. This is due to an electric 

 tension of the entire nerve, induced by the passage of a current through 

 a portion of its extent. This condition is called electrotonus. There 

 is also a peculiar condition of that portion of the nerve near the anode, 

 differing from the condition of the nerve near the cathode. Near the 

 anode the excitability of the nerve is diminished, and this condition is 

 called anelectrotonus. Near the cathode the excitability is increased, 

 and this condition is called catelectrotonus. These phenomena have 

 been the subject of extended investigation by electro-physiologists ; and 

 although the conditions are not to be included in the physiological prop- 

 erties of the nerves, they have considerable pathological and therapeutic 

 importance. It is well known, for example, that electricity often is one 

 of the most efficient agents at command for the restoration of the 

 properties of nerves affected 'with disease ; and the constant current 

 has been extensively and successfully used as a therapeutic agent. The 

 constant current, in restoring the normal condition of nerves, must influ- 

 ence, not only that portion included between the poles of the battery, 

 but the entire nerve ; and the electrotonic condition, with its modifica- 

 tions, in a measure explains how this result may be obtained. 



The electrotonic condition is marked in proportion to the excitability 

 of the nerve, and it is either absent or very feeble in nerves that are 

 dead or have lost their excitability. If a strong ligature is applied to 

 the extrapolar portion of a nerve, or if the nerve is divided and the cut 

 ends are brought in contact with each other, the electrotonic condition is 



1 The current described above was called by Du Bois-Reymond the "current of rest." 

 Some writers now adopt the view of Hermann that this current does not exist in normal muscle 

 or nerve but is due to injury of the tissue. 



