6oo A MANUAL OF PHYSIOLOGY 



It is impossible under these circumstances to take account 

 of the direction of a current in a nerve, or to connect direction 

 with any specific effect. When we place one of the electrodes 

 over the nerve and the other at a distance, the law of con- 

 traction only appears in a disguised form ; for since a 

 kathode and an anode exist at each pole, there is, with a 

 current of sufficient strength (' strong current')? excitation at 

 each both at make and break. The negative make-contrac- 

 tion is, however, stronger than the positive, for the excitation 

 corresponding to the latter arises at the secondary kathodic 

 surface, where the sheaf of current-lines spreading from the 

 positive electrode passes out of the nerve. Now, this is 

 much larger than the primary kathodic surface, through 

 which the narrow wedge of stream-lines passes to reach the 

 negative electrode, and the current density at the latter is 

 accordingly much greater. The positive break-contraction 

 is, for a similar reason, stronger than the negative. 



With a ' weak ' current, the only contraction is a closing 

 one at the kathode ; with a ' medium ' current there are 

 both opening and closing contractions at the positive 

 pole, and a closing but no opening contraction at the 

 negative. 



The conductivity of the nerve, as we have seen in various 

 examples, is not necessarily altered in the same sense as 

 the excitability. In the neighbourhood of the kathode it 

 is easier to cause excitation than in the normal nerve (in- 

 creased excitability), but it is less easy for an excitation set 

 up elsewhere to pass through (diminished conductivity). 

 Change of temperature seems also, for stimuli of not very 

 short duration, to act in the opposite way on these two 

 properties of nerve. Carbon dioxide appears to depress 

 the excitability without affecting the conductivity, and 

 alcohol to have the contrary effect (Gad and Sawyer). 

 Munk found that in a dying sciatic nerve certain points may 

 be quite inexcitable to the strongest stimuli, while weak 

 stimulation of points lying nearer the central end may cause 

 muscular contraction. These facts seem to show that the 

 process by which the nerve-impulse is propagated is not the 

 some as that by which it is originated and therefore is not 



