THE MOLECULAR THEORY OF EXCITATION 609 



mission, if the molecules in its path are already incipiently 

 orientated, so that the incident stimulus finds them pre- 

 disposed to respond in that direction and to transmit the 

 excitation. Hence the kathodic effect is more easily trans- 

 mitted through a tract which is in a state of K-tonus, 

 whereas it is retarded or inhibited under A-tonus. We shall 

 now study the corresponding effect in magnetic conduction. 

 Normal excitation in these experiments, it should be remem- 

 bered, is taken as that which is brought about by the north- 

 polar or K-effect. If there is a tonic coil, T, surrounding one 

 arm of the balance, then, by sending a permanent current of 

 moderate intensity round the coil, in one direction or the 

 other, we may induce at will, in that arm, either K-tonus 

 r A-tonus. The molecular disposition induced by the 

 stimulus, and by K- and A-tonus respectively, will be under- 

 stood from the diagrammatic representation given in fig. 372. 

 cal variation of excitability at E may be induced by 

 bringing near to it either the north 

 or south pole of a permanent 

 magnet M. 



I shall now exhibit the enhance- 

 ment or depression of magnetic 

 conductivity by K- or A-tonus. A 



balanced record is first obtained, 



,, - j j .1 FIG- 374- Effect of K- and 



and K-tonus then induced in the A-Tonus on Magnetic Con- 

 right arm. Successive K-make ex- duction 

 citations are now applied, starting % 

 from the centre of the balance at wards the effect of K-tonus 

 S, and proceeding onwards to left 



blU 



- 



and right simultaneously. The series with resultant 



. down-responses, show de- 



resulting responses are recorded, presskm by A-tonus. 



records of the break-effect being 



avoided by timely interruptions of the galvanometer-circuit. 

 It will be seen (fig. 374) from the upsetting of the balance in 

 an upward direction, that K-tonus has induced enhancement 

 of conductivity in the right arm. On now causing A-tonus, 

 by reversing the current in the enclosing tonic coil, T, a de- 



