ELECTR O-PHYSIOL OGY 64 1 



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, 

 equally possible, from what we know of the enormous electrical resist- 

 ance of the physiological envelopes of such cells as the red blood 

 corpuscles (p. 35), that the great transverse resistance of nerve, and 

 indeed the electrotonic currents, may be due in part if not wholly to 

 the true resistance of one or more of its envelopes (perhaps the 

 medullary sheath). Examples of such differences of resistance even 

 in the fluid constituents of one and the same animal structure are 

 not wanting. For instance, the resistance of the yolk of a hen's egg 

 may be three times greater than that of the white. 



The electrotonic currents cannot spread beyond a ligature ; they 

 are stopped by anything which destroys the structure of the tissue ; 

 they are affected by various reagents. But this does not prove that 

 they are other than physical in origin, for what destroys the structure 

 of the tissue or modifies its molecular condition may destroy or 

 diminish its capacity for polarization, or alter its electrical resistance. 



There are, however, certain facts which indicate that physiological 

 factors, as well as physical, are concerned. While the currents 

 obtained from core-models show a general resemblance to the 

 electrotonic currents of medullated nerve, there is one significant 

 difference : in the former the katelectrotonic and anelectrotonic 

 currents are of equal intensity ; in the latter the anelectrotonic pre^ 

 ponderates. The most probable explanation is that the anelectro- 

 tonic current of medullated nerve is made up of two distinct electrical 

 effects, one physiological in nature, the other dependent merely on 

 the structure and physical properties of the fibres, while the kat- 

 electrotonic current is wholly physical. It is in favour of this 

 hypothesis that under the influence of ether, which abolishes the 

 physiological functions of nerve, the anelectrotonic current diminishes 

 till it becomes equal to the katelectrotonic. Non-medullated nerves, 

 in which the conditions for physical electrotonus, if present at all, 

 are only feebly developed, and which exhibit no katelectrotonic 

 current, or only a very weak one, show an anelectrotonic current, 

 which is abolished by ether, and seems to represent the physiological 

 portion of the anelectrotonic current of medullated nerve. 



Physical electrotonus must be distinguished from the changes of 

 excitability produced by the constant current, to which the name of 

 electrotonus is also sometimes given. For although the decline in 

 the intensity of the electrotonic currents, as we pass away from the 

 electrodes, has its analogue in the distribution of the electrotonic 

 changes of excitability, and there are other facts which suggest a 

 relation between the two, we are ignorant of the real nature of this 

 relation. 



Effects of Stimulating a Polarized Nerve. Stimulation of the 

 nerve while the polarizing current is flowing causes in general in the 

 extrapolar regions a negative variation of the electrotonic current^ but 

 in the intrapolar region a positive variation. The latter is undoubtedly 



41 



