250 PHYSIOLOGY CHAP. 



The same law of contraction applies to sensory or afferent 

 nerves. In this case the reflex muscular response is taken as 

 the measure of excitation in the nerve. Here the results must 

 of course be inverted, the reflex contractions excited from sensory 

 nerves with ascending and descending currents following the 

 law of motor nerves for the descending or ascending currents 

 respectively. 



The expressions adopted in the formula of the law of con- 

 traction, of weak, medium, strong, or very strong currents, have 

 only a relative value, since the local phenomena of excitation due 

 to polar changes depend not only on the strength, direction, and 

 duration of the current, but also on the initial excitability and 

 the length of nerve traversed by the current. It has been shown 

 that polar electrical stimulation is more effective as the electrodes 

 are further apart, because the changes in the equilibrium of the 

 nerve are so much the more difficult 'to compensate. 



All the phenomena of Pfliiger's law come off equally well with 

 tripolar excitation of the nerve, as in Fig. 160. The nerve is even 

 more sensitive to this form of stimulation, probably owing to the 

 larger area of the intrapolar tract, so that currents which were 

 ineffective with ordinary bipolar contacts may become effective. 



There may be exceptions to Pfliiger's law owing to the influence 

 of accessory factors. Such are the local alterations of excitability 

 due to the effect of temperature, to salt solutions, to interference or 

 coincidence of the polarising current and the demarcation current, 

 etc. When, for instance, the kathode is close to the section in a 

 freshly divided nerve, a break contraction can be obtained not 

 only with medium, but also with weaker currents, which are 

 usually ineffective. This is because in such a case the descending 

 break current summates with the demarcation current, which is 

 also descending. When, on the contrary, the two currents are 

 opposite in direction, the effects are neutralised. It can, in fact, 

 be demonstrated experimentally that a vigorous demarcation 

 current is able to annul the exciting action of a weak polarising 

 current in the opposite direction (Hering). 



The polarisation after-effect, which appears in the nerve after 

 the passage of a polarising current of sufficient strength and 

 duration, may both at make and at break render another current 

 in the same direction effective when the latter is too weak to 

 produce any excitation alone. The break contraction resulting 

 in this case may be taken as a proof of the fact that the disappear- 

 ance of anelectrotonus is as capable of arousing excitation as the 

 appearance of katelectrotonus. 



The polarisation after-current on the passage of a strong 

 polarising current may itself cause a prolonged excitation 

 expressed by the persistent contraction of the muscle. This 

 phenomenon is known as Hitter's opening tetanus. It is seen 



