ELECTROTONIC CURRENTS 805 



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 resistance, this 

 has been adduced as evidence of the great capacity of nerve for polar- 

 ization by a current passing across the fibres. It is, however, probable, 

 from what we know of the high electrical resistance of the physiological 

 envelopes of such cells as the red blood-corpuscles (p. 26), that the 

 great transverse resistance of nerve, and indeed the electrotonic currents, 

 are 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 specific 

 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 preponderates. The most probable ex- 

 planation is that the anelectrotonic 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 katelectrotonic 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 kateletro tonic. 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 anelectro- 

 tonic current of medullated nerve. 



A nerve may be stimulated by an electrotonic current produced 

 in nerve-fibres lying in contact with it. A well-known illustration 

 of this is the experiment known as the paradoxical contraction 

 (Practical Exercises, p. 816). 



The current of action of a nerve can also stimulate another nerve 

 when the excitability of both is greater than normal, as is the case 

 in the nerves of frogs kept in the cold. This comes under the head 

 of secondary contraction. But the best-known form of secondary 

 contraction is where a nerve, placed on a muscle so as to touch it in 

 two points (Fig. 296), is stimulated by the action-current of the 

 muscle, and causes its own muscle to contract. A secondary tetanus 



* 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 disproportion 

 between the longitudinal and transverse resistance of the nerve fibres them- 

 selves is probably much greater than this. 



