604 NERVOUS SYSTEM. 



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

 The phenomena thus produced have been most elaborately studied by Pfluger, who 

 farther recognized a peculiar condition of that portion of the nerve near the anode, or 

 positive pole, differing from the condition of the nerve near the cathode, or negative pole. 

 Near the anode, the excitability of the nerve is diminished, and this condition has been 

 called anelectrotonus. Near the cathode, the excitability is increased, and this condition 

 has been called catelectrotonus. 



These varied phenomena have been the subject of extended investigation by electro- 

 physiologists ; and, although they are not to be ranked among the physiological properties 

 of the nerves, they have considerable pathological and therapeutical importance. It is 

 well known, for example, that electricity is one of the most efficient agents at our com- 

 mand for the restoration of the functions of nerves affected with disease ; and the con- 

 stant current has, particularly of late, been extensively and successfully used as a thera- 

 peutical 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 electrotonic condition, with its modifications, explains how this result 

 may be obtained. Undoubtedly the sensory nerves are affected as well as the motor, 

 although we have as yet but little positive information upon this point. A knowledge of 

 the fact that the constant current diminishes the excitability of the nerve near the anode 

 (anelectrotonus) and increases it near the cathode (catelectrotonus) may become important 

 in determining the direction of the current to be employed in different cases of disease. 



In the present condition of the subject of electro-physiology, it will be unnecessary to 

 do more than to indicate, as clearly and simply as possible, the laws of the phenomena 

 attending the passage of a constant current through nerves, as far as they have been 

 definitively ascertained. 



The phenomena of electrotonus are very simple ; and it is only when we attempt to 

 construct a theory to account for these phenomena that the subject becomes obscure. 

 Suppose, for example, that a nerve be exposed in a living animal or in one just killed, 

 and a galvanic current be applied from a Grove's battery, in which about twelve square 

 inches of zinc are exposed to the action of a liquid containing one part of ordinary sul- 

 phuric acid to eight of water. A delicate galvanometer applied to the nerve either above 

 or below the poles will indicate a decided current, much more intense than the tranquil 

 nerve-current between the exterior and the cut surface. This electrotonic condition exists 

 so long as the galvanic current is continued ; and, as has been shown by Matteucci in 

 operating upon the higher animals rabbits, dogs, fowls, and sheep when the galvanic 

 current has been sufficiently powerful and prolonged, the electrotonic condition persists 

 for a certain time after the stimulus has ceased. As we have seen that the muscular 

 contraction following galvanic stimulation of a nerve is powerful in proportion to the 

 extent of nerve included between the poles of the battery, so the electrotonic condition 

 increases in intensity with the length of the nerve subjected to the constant current; 

 provided, always, that the strength of the current be slightly increased to compensate the 

 enfeebling action due to the resistance in the increased length of the circuit. 



We do not propose to discuss fully the various theories that have been advanced in 

 explanation of the phenomena of electrotonus. Matteucci has made a series of interesting 

 observations upon conductors formed of very fine wires, one of platinum and the other 

 of amalgamated zinc, covered with cotton thread soaked in a neutral solution of sulphate 

 of zinc. The experiments were then arranged so as to operate first with the platinum 

 wire and afterward with the zinc, by passing a galvanic current through a small portion 

 of the conductor, in the same way as it is passed through a portion of a nerve. He found 



