494 NERVE. 



although the external polarisation is comparatively insignificant, there 

 is every indication of the formation of electrolytic ions within the nerve ; 

 this must be termed internal polarisation. It is dependent upon the 

 structural and physiological properties of living nerve, and is especially 

 marked in the case of medullated nerve fibres. It can be produced in 

 any structure consisting of a better conducting continuous core, and 

 a worse conducting moist envelope, such as the model devised by 

 Hermann, in which a platinum wire lies within a tube of sulphate of 

 zinc or saline solution. On closing the current, the internal polarisation 

 changes are most intense at the poles (anode and cathode), but spread 

 into the adjoining intrapolar and extrapolar regions. 



These polarisation products are themselves electromotive, so that 

 the nerve is in a peculiar condition termed by its discoverer, du Bois- 

 Keymond, electrotonus. 1 It would' be unprofitable at this stage to enter 

 in any detail into these electrical effects, the foregoing reference being 

 intended merely to introduce the excitability changes which are produced 

 at the same time as these alterations in electrical state, and which have 

 been therefore termed the electrotonic alterations in excitability. 



Polar changes in excitability. Pfliiger's 2 work on this subject 

 culminated in a series of general conclusions, deduced from a study of 

 nerve, so careful and exhaustive, that the results have stood the test 

 of time as few other physiological data have done. Subsequent work 

 has extended but not materially modified the original statements, which 

 may be briefly summed up as follows : 



1. Under the influence of a constant current flowing through a nerve, 

 there is an increase in the nerve excitability, at and near the negative 

 pole (cathode), a decrease at and near the positive pole (anode). 



2. On the cessation of the current these changes are reversed, the 

 cathode being the seat of a fall, and the anode that of a rise in 

 excitability. 



3. The alterations in excitability are most intense at the poles, but 

 spread, diminishing with the distance into the intrapolar and extra- 

 polar regions. 



4. At some point in the intrapolar region the boundary between 

 the two polar extensions is reached ; this point is therefore unaffected, 

 and is termed the indifference point. 



5. The excitability changes are true for all forms of stimulation, 

 electrical, mechanical, or chemical, and for both efferent and afferent 

 nerves. 



The experimental proof of the first two propositions is readily 

 effected in a muscle-nerve preparation, by arranging so that a uniform 

 stimulus of minimal exciting intensity is applied to the nerve just 

 beyond the pole of the polarising current which is nearest to the muscle. 

 If this pole be made the anode, the stimulus becomes, during closure, 

 ineffectual ; if the cathode, it becomes of maximal exciting intensity, and 

 the alteration persists during the whole time of the closure of the 

 current. A similar method demonstrates the anodic rise and the 

 cathodic fall of excitability immediately after closure. The pronounced 

 character of the alterations at the poles themselves can be ascertained 

 by using such an arrangement as that employed by Griinhagen, in 

 which a uniform exciting stimulus, the break or the make induced 



1 E. du Bois-Keymond, " Untersuchungen," loc. cit. 



2 "Electrotonus," Berlin, 1859. 



