6 10 COMPARATIVE ELECTRO-PHYSIOLOGY 



pression of conduction is induced, as shown by the upsetting 

 of the balance in a downward direction. 



We next turn to the question of the variation of conduc- 

 tivity induced by K-tonus, when moderate or excessively 

 strong ; and it is here important to forecast from theoretical 

 considerations what is to be expected under varying intensities 

 of the polarising force. It is easy to understand that moderate 

 K-tonus, inducing an incipient orientation of the molecules, 

 will predispose them to easy upset in a particular direction, 

 thus greatly facilitating the transmission of excitation from 

 point to point Thus a moderate K-tonus will enhance con- 

 ductivity. But if the K-tonus in question be excessive, so that 

 the molecules are already distorted to their maximum position, 

 incident stimulus can then induce no further change, and under 

 such circumstances there can be little transmission, Hence, 

 under increasing intensity of K-tonus, we may expect to obtain 

 increasing conductivity up to a certain point. But, beyond 

 this, the conductivity will be decreased, or even actually 

 inhibited. 



These anticipations are seen fully verified in the accom- 

 panying record (fig. 375), which shows the opposite effects 



on conductivity of moderate 

 and strong K-tonus. The 

 upsetting of the balance 

 in an upward direction, 

 K, shows the effect of 

 moderate K-tonus. Strong 

 K-tonus was next applied, 



FIG. 375. Opposite Effects of K-Tonus ., , , , cr f ... 



whin mo4erate and strong Wlth the effect of upsetting 



the balance in the opposite 



direction, K'. Thus we see that, while under moderate 

 K-tonus the conductivity is enhanced, under a much greater 

 intensity it becomes depressed. This will, I think, be found 

 to explain a somewhat anomalous occurrence, which has 

 been observed in regard to the conduction of excitation 

 through a kathodic region in nerve-and-muscle preparation. 

 A stimulus applied on the extra-polar region in nerve is 



