ELECTROTONICITY OF NERVES. 227 



citing currents, they are supposed by Du Bois-Reymond to be so acted 

 on, as to become dipolar. On the former supposition, of the nerve 

 consisting of rows of single peripolar molecules, a shifting or alteration 

 of their polarity is assumed to occur ; but, on the latter supposition, 

 every other molecule is imagined to undergo a change in its polarity, 

 or seems to turn half-way round, as shown in 6, Diagram F. This new 

 condition of the nerve, as regards the electrical state of its molecules, 

 is spoken of as its electrotonicity. 



Diagram F. 



Diagram F shows a further view of the supposed electrical constitution of the nerve molecules, and also 

 of the change in them, supposed to be produced in the state of electrotonus, or the electrotonic condition 

 of nerves, a shows the supposed double constitution of the nerve molecules, each pair making up a com- 

 posite peripolar molecule in the normal nerve, b shows how. by the half revolution of every other mole- 

 cule, the whole chain may obey the direction of an exciting current or stimulus, and become electrotonic, 

 i.e., its molecules may assume the electrical character of those in any ordinary galvanic circuit. The 

 arrows show the direction of a current outside such rows of molecules, as it would pass through a galvano- 

 metric circuit connected with them. 



This electrotonic condition is so important, that we are justified in 

 again comparing the effects of electricity on muscle and nerve, so as 

 clearly to impress upon the mind this distinction between those two 

 important tissues. When a muscle is excited to contract by any 

 kind of stimulus, its normal electrical current is simply diminished ; 

 even if the stimulus be itself electrical, no other result seems to hap- 

 pen, and no further alteration of the electrical condition of the muscle 

 takes place. But in regard to nerve, besides the diminution in the 

 proper nerve current, which takes place as the result of the applica- 

 tion of all kinds of stimuli, the electrical stimulus, when applied to a 

 part of a nerve, by means of a continuous galvanic current, not merely 

 diminishes the normal current through that part, but induces a par- 

 ticular and obedient change in the electrical polarity of the nerve- 

 molecules of the neighboring parts also, and thus throws not only the 

 excited portion, but the neighboring portions likewise, above and 

 below, into the electrotonic state. It is more marked when the ex- 

 citing current is strong ; and less so, the greater the length of the 

 excited portion of the nerve. It is also more perfect in the portions 

 of nerve immediately preceding or succeeding the part excited ; it be- 

 comes weaker, as the distance from that part increases upwards or 

 downwards, and at last it disappears in both directions, the normal 

 nerve current pursuing its usual course. 



But not only is the occurrence of electrotonicity in a nerve, a character 

 which distinguishes that tissue from muscle ; but what is even more 

 important, certain changes in the characteristic physiological property 

 of the nerve, that is to say, in its excitability to stimuli, or its con- 

 ducting power, simultaneously take place, thus, when an exciting 

 current, Diagram G, B, is passed through a portion, a, of a nerve, 



