VARIATIONS OF THE EXCITABILITY DURING ELECTROTONUS. 565 



The same effect occurs in the movement of water, which results in that condition known as 

 imbibition of a body. We must remember, that at the demarcation surface of an injured nerve 

 or muscle, imbibition takes place ; that also at the contracted parts of a muscle imbibition of 

 fluid occurs ( 227, II.) ; and that during secretion there is a movement of the fluid particles. 



In plants, electrical phenomena have been observed during the passive bending of vegetable 

 parts (leaves or stalks), as well as during the active movements which are associated with the 

 bending of certain parts, e.g., as in the mimosa and dionsea {Burdon- Sanderson). These 

 phenomena are perhaps explicable by the movement of water which must take place in the 

 interior of the vegetable parts {A. G. Kunkel). The root cap of a sprouting plant is negative 

 to the seed coverings {Hermann) ; the cotyledons positive to the other parts of the seedling 

 {MiiUer-Hettlingen). In the incubated hen's egg, the embryo is + , the yelk - {Hermann and 

 v. Gendre). 



335. ELECTRONIC ALTERATION OF THE EXCITABILITY. Cause of 



Electrotonus. If a certain stretch of a living nerve be traversed by a constant 

 electrical {"polarising " ) current, it passes into a condition of altered excitability 

 (Bitter, 1802, and others), which du Bois-Keymond called the electro tonic condition, 

 or simply electrotonus. This condition of altered excitability extends not only over 

 the part actually traversed by the current, intrapolar portion, but it is communi- 

 cated to the entire nerve, i.e., to the extrapolar portions. Pfliiger (1859) discovered 

 the following laws of electrotonus : 



At the positive pole or anode (fig. 408, A) the excitability is diminished this 

 is the region of anelectrotonus ; at the negative pole or cathode (K) it is increased 

 this is the region of cathelectrotonus. The changes of excitability are most 

 marked in the regions of the poles themselves. 



Indifferent Point. In the intrapolar region a point must exist where the 

 anelectrotonic and cathelectrotonic regions meet, where therefore the excitability is 

 unchanged; this is called the indifference or neutral point. This point lies 

 nearer the anode (i) w T ith a weak current, but with a strong current nearer the 

 cathode (i t ) ; hence, in the first case, almost the whole intrapolar portion is more 

 excitable ; in the latter, less excitable. [Expressed otherwise, a weak current 

 increases the area over which the negative pole prevails, while the reverse is the 

 case with a strong current. Or in the intrapolar region, the diminution of excita- 

 bility extends as the strength of the current increases, or to put it otherwise, with 

 an increasing strength of current, the indifferent point moves from the positive to 

 the negative pole.] Very strong currents greatly diminish the conductivity at the 

 anode, and indeed may make the nerve completely incapable of conduction at this 

 part. 



At the cathode also, but only after the polarising current has passed for some time through 

 the nerve ( Werigo), the excitability is diminished, and the nerve in this area is rendered 

 incapable of conduction {Grilnhagen). 



Extrapolar Region. The extrapolar area, or that lying outside the electrodes, 

 is greater the stronger the current. Further, with the weakest currents, the extra- 

 polar anelectrotonic area is greater than the extrapolar cathelectrotonic. With 

 strong currents this relation is reversed. 



Fig. 408 shows the excitability of a nerve {N, n) traversed by a constant current in the 

 direction of the arrow. The curve shows the degree of increased excitability in the neighbour- 

 hood of the cathode {K) as an elevation above the nerve, diminution at the anode {A) as a 

 depression. The curve m, 0, i, jo, r, shows the degree of excitability with a strong current ; 

 c i ft in h, k, with a medium current ; lastly, a, b, i, -c, d, with a weak current. 



The electrotonic effect increases with the length of the nerve traversed by the current. The 

 changes of the excitability in electrotonus occur instantly when the circuit is closed, while 

 anelectrotonus develops and extends more slowly. Cold diminishes electrotonus {Hermann and 

 v. Gendre). 



When the polarising current is opened or broken, at first there is a reversal of 

 the relations of the excitability, and then there follows a transition to the normal 

 condition of excitability of the passive nerve (Pfliiger). At the very first moment 

 of closing, Wundt observed that the excitability of the whole nerve was increased. 



