156 ELECTRO-PHYSIOLOGY CHAP. 



alterations of excitability in the nerve, so that it is no wonder 

 if, under certain conditions, there are apparent exceptions from 

 the rule. Apart from the well-known tendency of " cooled frogs " 

 to sustained tetanic excitation, which renders them almost en- 

 tirely unsuitable to the demonstration of the law of contraction, 

 we have to remember the influence exercised by the proximity of 

 an artificial cross-section, not merely upon general excitability, 

 but also upon the mode of action of the battery current. 



As regards the former, the raised excitability at the cross- 

 section of a medullated nerve must be regarded essentially as 

 a consequence of katelectrotonus, produced within a certain 

 tract from the cross-section, by internal short-circuiting of the 

 nerve-current, a fact to which we shall return later. We must 

 first consider the influence, of the transverse section upon the polar 

 action of current. 



If unpolarisable electrodes are applied to the cut end of a 

 motor nerve of a frog, so that the lower electrode in contact with 

 the " long section," and the upper resting on the cross-section, 

 are sufficiently approximated, the intrapolar tract being very short, 

 the effect, even with minimal currents, invariably corresponds 

 with the third stage of the law of contraction. At a some- 

 what greater distance of the peripheral electrode (intrapolar 

 tract = 12 cm.) the effect of excitation with weak currents 

 is, on the other hand, a closure twitch with ascending, a closure 

 and an opening twitch with descending direction of current. In 

 this connection we have also the experiment of Heidenhain (29), 

 who divided the nerve between the electrodes and closed up the 

 cut ends again ; whereupon, if the incision was made sufficiently 

 near the myopolar electrode, the activity of the latter alone 

 persisted. The zone of depressed excitability in the region of the 

 cut surface in nerve appears to be considerably greater in warm- 

 than in cold-blooded animals. At least the experiment comes off 

 with a greater distance of electrodes in the first case than in the 

 last. If the sciatic nerve of a mammal or bird is exposed in its 

 full extent, and then divided (after previously determining the 

 exclusive effect of closure with not excessively strong ascending or 

 descending currents, at the electrode proximal to the centre), 

 there will, with the electrodes close together (about 1 cm.), 

 under the same conditions as before, be a closure twitch only witli 

 descending direction of current, an opening twitch with ascending 



