ELECTROTONUS. 535 



muscular contraction itself, by the fact that the same phenomena occur when 

 the first muscular contraction is produced by mechanical or chemical excita- 

 tion of the nerve. 



Galvanic Current from the Exterior to the Cut Surface of a Nerve. Be- 

 fore studying certain phenomena presented in nerves of which a portion is 

 subjected to the action of a constant galvanic current, it is important to note 

 the fact that there exists in the nerves, as in the muscles, a galvanic 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 muscular current (Matteucci). The existence of the nerve-current has, 

 as far as is known, no more physiological significance than the analogous 

 fact observed in the muscular tissue. Galvanic currents also exist in the 

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

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



Electrotonus, Anelectrotonus and Catelectrotonus. When a constant gal- 

 vanic 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 (Du 

 Bois-Reymond). While in this state, the nerve will deflect the needle of a 

 galvanometer, and its excitability is modified. The deflection of the needle 

 in this instance is not due to the normal nerve-current, for it occurs when 

 the galvanometer is applied to the surface of the nerve only. It 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 ane- 

 electrotonus. Near the cathode the excitability is increased, and this condi- 

 tion is called catelectrotonus (Pfliiger). These phenomena have been the 

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

 conditions are not to be included in the physiological properties of the 

 nerves, they have considerable pathological and therapeutical importance. 

 It is well known, for example, that electricity is often one of the most effi- 

 cient 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 therapeutical agent. The constant current, in restoring 

 the normal condition of nerves, must influence, not only that portion included 

 between the poles of the battery, but the entire nerve ; and the electro tonic 

 condition, with its modifications, 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 entirely absent or extremely feeble in nerves that 

 are dead or have lost their excitability. If a strong ligature be applied to the 

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

 brought in contact with each other, the electrotonic condition is either not 

 observed or is very feeble. These facts show that the phenomena of electrot- 

 onus depend upon the physiological integrity of nerves. A dead nerve or 



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