660 IRRITABILITY OF NERVE AND MUSCLE IN ELECTROTONUS. 



like manner been determined that everywhere dying and active contents are 

 negative to resting normal contents. 



Attention may yet be called to the following facts. If water passes through 

 a capillary space an electrical movement takes place in the same direction. So 

 also the advance of water in the capillary interstices of inanimate structures 

 (pores of a porcelain plate) is attended with an electrical movement in the same 

 direction as the stream of water. Exactly the same conditions prevail in the 

 movement of water that causes the swelling of a body. Landois has pointed 

 out that imbibition and swelling take place at the demarcation-surface of an 

 injured muscle or nerve; further that swelling occurs also at the contracted por- 

 tion of a muscle in consequence of the absorption of fluid, and that in the process 

 of secretion movement of fluid takes place from the blood into the glandular 

 cells and from these to the excretory ducts. Mention should finally be made of 

 the fact, as H. Munk found, that, at the moment of closing the current at the 

 anode and beyond, loss of water and increase in resistance take place in the nerve; 

 and at other situations and beyond the kathode the reverse. The total resistance 

 of the distance traversed diminishes at first, then increases with accelerated rapid- 

 ity. On opening the current neutralization of this difference rapidly takes place. 

 In plants electrical phenomena are observed both on passive bending of portions 

 of plants, as of the leaves or the stems, and also in active movements associated 

 with bending of parts of the plants, for example in the movements of the mimosa, 

 the dionea, and others. These electromotor effects are also to be explained in 

 all probability by the movement of water in the parts of the plant, which must 

 take place in their interior on movement. The tip of the root of germinating 

 plants is negative with respect to the seed-covering, the cotyledons positive with 

 relation to all other portions of the seedling. In the incubated bird's egg the 

 embryo is positive, the yolk negative. 



ALTERED IRRITABILITY OF NERVE AND MUSCLE IN ELECTRO- 

 TONUS. 



If a living nerve is traversed throughout a definite length by a 

 constant electrical (polarizing) current it passes into the condition of 

 altered irritability that is designated the electrotonic state or simply 

 electrotonus. The condition of altered irritability extends not alone 

 over the traversed (intrapolar) distance, but is communicated to the 

 entire nerve. 



Pfluger has discovered the following law of electrotonus. At the 

 positive pole or anode (Fig. 232, A) the irritability is diminished and 

 anelectrotonus prevails. At the negative pole or kathode (K) it 

 is increased, and the increase in irritability prevailing here is desig- 

 nated katelectrotonus. These alterations in irritability are most 

 pronounced near the poles. 



In the intrapolar segment there must naturally be a point where 

 anelectrotonus and katelectrotonus coincide and where therefore the 

 irritability is unaltered. This point is designated the indifferent point. 

 It is situated in the case of feeble currents near the anode (i), in that 

 of strong currents, however, near the kathode (i^}\ therefore in the 

 first instance almost the entire intrapolar segment is more irritable and 

 in the latter less irritable. Exceedingly strong currents greatly dimin- 

 ish the conducting power at the anode and they may even render the 

 nerve wholly incapable of conducting. 



Also at the kathode, but only after the current has been for some time flowing 

 through the nerve, the irritability is diminished and the nerve becomes incapable 

 of conducting. 



Beyond the electrodes (extrapolar) the area of altered irritability 

 is the more extensive the stronger the current. Further, the extent of 



