NOTES AND ADDITIONS. 309 



current does not exceed a certain strength, it can excite; 

 but when it is strong it is ineffective, since the closing of 

 strong ascending currents is always ineffective. If, however, 

 the current is made yet stronger, it may again become effec- 

 tive, because the opening portion of the current can now, in 

 spite of its retarded course, cause an irritation. This gap 

 (Liicke) in the action was observed by Tick, and afterwards 

 by Tiegel. How far other causes besides those here ex- 

 plained combine to produce this peculiar phenomenon, we 

 cannot examine further here. 



8. ACTION OF TRANSVERSE CURRENTS. UNIPOLAR IRRITATION 



(p. 152). 



If a current is passed transversely through a nerve, that 

 is, in a direction at right angles to the long a,xis of the nerve 

 fibres, it has no effect. To effect the alteration in the posi- 

 tion of the nerve molecules which we regard as the cause 

 of the process of excitement, the current must, therefore, 

 pass in the longitudinal direction of the nerve. This is pro- 

 bably due to the peculiar electric forces of the nerve-par- 

 ticles, which are treated of in detail on page 215 et scq. Just 

 as an electric current if it flows parallel to a magnetic needle 

 deflects the latter, but has no such effect when it flows in a 

 direction at right angles to that of the needle, so the nerve 

 particles can only be disturbed from their quiescent position 

 by currents which run parallel to the axis of the nerve. If 

 the current is directed oblique!}' to the nerve fibre, it acts 

 but not so strongly as when it is parallel, and the degree of 

 the action decreases proportionately as the angle which the 

 current makes with the nerve-fibre approaches more nearly 

 to a right angle. 



The connection between the phenomena of electrotonus 

 and excitement of the nerve led us to believe that the excite- 

 ment takes place, not throughout the whole portion of the 

 nerve traversed by the current, but only in a part which on 



