728 A MANUAL OF PHYSIOLOGY 



of these observers that there is only one kind of electrical stimulus, 

 the kathodic, or make, has not been clearly established. 



Electrotonic Currents. If a current be passed from the 

 battery through a medullated nerve (Fig. 278) in the direction 

 indicated by the arrows, while a galvanometer is connected with 

 either of the extrapolar areas, as shown in the figure, a current will 

 pass through the galvanometer, in the same direction in the nerve 

 as the polarizing current, so long as the latter continues to flow. 



These currents are called electrotonic (in the kathodic region 

 katelectrotonic ; in the anodic, anelectrotonic}. The exact mode of 

 their production is obscure. Similar currents can be detected in 

 artificial models consisting of a good conducting core, and a badly 



conducting envelope ; 

 for example, a plati- 

 num wire in a glass 

 tube filled with satu- 

 rated zinc sulphate 

 solution, or a zinc 

 wire covered with 

 FIG. 278. DIAGRAM SHOWING DIRECTION OF THE cotton-wool soaked in 

 EXTRAPOLAR ELECTROTONIC CURRENTS. salt solution. In such 



+ is the anode and - the kathode of the models it appears to 

 polarizing current. be essential that there 



should be polarization 



(separation of ions) at the boundary between the core and the sheath 

 i.e., between the wire and the liquid, where the current passes 

 from the one to the other. 



A current led into the sheath tries, so to speak, to pass mostly by 

 the good conducting wire. If this is not polarizable if it is, e.g., a 

 zinc wire surrounded by saturated zinc sulphate solution there is 

 little or no spreading of the current outside the electrodes : it passes 

 at once into the core, and so on to the other electrode. If, however, 

 there is polarization when the current passes from the liquid into the 

 wire, as is the case in the platinum-zinc sulphate, or the zinc-sodium 

 chloride combinations, the stream spreads longitudinally in the 

 sheath since the polarization introduces a virtual resistance at the 

 surface of the wire, in comparison with which the difference in 

 resistance of an oblique and a direct transverse path through the 

 liquid becomes small. It has been supposed that in medullated 

 nerve a similar polarization takes place at the boundary between 

 some part of the nerve-fibre which may be called a core, and another 

 part which may be called a sheath for instance, between the axis- 

 cylinder and the medullary sheath, or between the latter and the 

 neurilemma. It is known that the electrical resistance of nerve in 

 the transverse direction is much greater (five to seven times)* than 

 the longitudinal resistance. Since a rapidly-established polarization 

 would, by the ordinary methods of measurement, appear as a resist- 

 ance, this has been adduced as evidence of the great capacity of 

 nerve for polarization by a current passing across the fibres. It 

 is, however, probable, from what we know of the high electrical 



* Since a part of the current is conducted by the connective tissue 

 and other structures lying between the nerve-fibres, and the longitudinal 

 and transverse resistance of these tissues may be supposed equal, the dis- 

 proportion between the longitudinal and transverse resistance of the 

 nerve-fibres themselves is probably much greater than this. 



