GENERAL PROPERTIES OF THE NERVES. 605 



that in this way he could produce a strong electrotonic current in the platinum wire, even 

 at a distance of more than three feet from the electrodes, while no such current was 

 observed in the zinc. He remarks that in the platinum wire " secondary polarities" are 

 produced very powerfully and rapidly, while these are not developed in the zinc. From 

 these experiments alone, it might seem that the phenomena of electrotonus are to be 

 explained entirely by the physical properties of the nerves as conductors of electricity ; 

 but various observations on the nerves under different conditions have conclusively proven 

 the contrary. All observers are agreed that the electrotonic condition is marked in pro- 

 portion to the excitability of the nerve, and it is either entirely absent or extremely feeble 

 in nerves that are dead or have lost their irritability. If a strong ligature be applied 

 to the extra-polar portion of the 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 conclusively that the phenomena of electrotonus 

 depend upon the physiological integrity of nerves. A dead nerve, or one that has been 

 divided or strongly ligatured, may present these phenomena under the stimulation of a 

 very powerful current (and then only to a slight degree), when the condition depends 

 upon the purely physical properties of the nerve as a conductor ; but there is no com- 

 parison between these phenomena and those observed in nerves that retain their physi- 

 ological properties. Were it otherwise, how could the physiological properties of a dis- 

 eased nerve be restored throughout its whole extent by a constant current passed through 

 a restricted portion, when the excitability of the nerve is only manifested at the clos- 

 ing or opening of the circuit? 



Anelectrotonus and Cat electrotonus. It is interesting to note that, when a portion of 

 a nerve is subjected to a moderately powerful constant current, the conditions of the 

 extra-polar portions corresponding to the two poles of the battery are entirely different. 

 Near the positive pole, or anode, the excitability of the nerve and the rate of nervous 

 conduction are diminished. If, however, we have a galvanometer applied to this portion 

 of the nerve, its electromotive power, measured by the deflection of the galvanometric 

 needle, is increased. On the other hand, near the negative pole, or cathode, the excita- 

 bility of the nerve is increased, as well as the rate of nervous conduction ; but the elec- 

 tromotive power is diminished. These facts, at least so far as they relate to the increase 

 of the excitability of the nerve near the cathode and its diminution near the anode, 

 are partially explained by Matteucci upon purely physical principles, depending upon 

 the electrolytic action of the current, as is shown by the following experiment : 



Two cups are filled, the one with a very feebly alkaline solution, and the other with 

 an equally weak acid fluid. A number of galvanoscopic frogs' legs are then rapidly pre- 

 pared, of which one-half the number is plunged in the alkaline and one-half, in the acid 

 fluid, for from thirty seconds to one or two minutes. The parts are then removed from 

 the liquids and are carefully washed and dried in bibulous paper. By touching the 

 nerves with a strong solution of common salt, which is a powerful excitant for the ner- 

 vous irritability, the nerves that had been exposed to the alkaline solution produced 

 more powerful and prompt contractions than those exposed to the acid. Now, the elec- 

 trolytic action of a constant current tends to the accumulation of hydrogen and an alkali 

 near the cathode, and of oxygen and an acid near the anode ; and by this fact, Matteucci 

 explains the increase of excitability in catelectrotouus and the diminished excitability in 

 anelectrotonus. As regards this question, we have only to say, as in the case of gen- 

 eral electrotonus, that the conditions are susceptible of a partial explanation upon purely 

 physical grounds ; but precisely how far the unexplained physiological properties of the 

 nerves are involved, it is impossible to say. 



Neutral Point. The anelectrotonic condition, on the one hand, and the catelectro- 

 tonic condition at the other pole of the battery, are marked in extra-polar portions of 



