Currents on Transmission of Excitation. 



501 



the normal. These anticipations are fully supported by results of experi- 

 ments. Sub-maximal stimulus of equi-alternating induction shock was 

 directly applied to the muscle and records taken of (1) response under normal 

 condition without any current, (2) response under heterodromous current, the 

 tendon being the anode, and (3) response under homodromous current, the 

 tendon being now made the kathode. It was thus found that under 

 heterodromous current the excitability of the muscle was depressed, and 

 under homodromous current the excitability was enhanced. 



The effect of current on response to direct stimulation is thus opposite to 

 that on response to transmitted excitation, as will be seen in the following 

 Table. 



Table VI. — Influence of Direction of Current on Direct and Transmitted 



Effects of Stimulation. 



Direction of current. 



Transmitted 

 excitation. 



Direct stimulation. 



Heterodromous current 

 Homodromous current 



Enhanced response 

 Depressed response 



Depressed response. 

 Enhanced response. 



The passage of a current, therefore, induces opposing effects on the con- 

 ductivity of the nerve and the excitability of the muscle, the resulting 

 response being due to their differential actions. Under heterodromous current 

 a more intense excitation is transmitted along the nerve, on account of 

 induced enhancement of conductivity. But this intense excitation finds the 

 responding muscle in a state of depressed excitability. In spite of this the 

 resulting response is enhanced (fig. 8). The enhancement of conduction under 

 heterodromous current is, in reality, much greater than is indicated in the 

 record. Similarly under homodromous current the depression of conduction 

 in the nerve may be so great as to cause even an abolition of response (fig. 9), 

 in spite of the enhanced excitability of the muscle. The actual effects of 

 current on conductivity are, thus, far in excess of what are indicated in the 

 records. 



The two factors, namely, the induced variation of the conductivity of the 

 nerve and the excitability of the muscle, being antagonistic, certain effects 

 may be predicted when the relative values of the two are changed in definite 

 ways. Let us first consider the effect of diminishing the factor of conductivity 

 of nerve to zero, by bringing the stimulator near the muscle, this being tanta- 

 mount to direct stimulation. The result is seen in the third column of the 

 Table given above. We may next increase the value of the conductivity factor 



2 E 2 



