194 ELECTRO-PHYSIOLOGY CHAP. 



\ as well as excitation proceeds at make from the kathode, at break 

 from the anode. 



It suffices to divide the claw-nerves in the proximity of the 

 central electrode, or to kill a certain portion of them ab initio 

 by warming (dipping the limb of the claw in hot water almost 

 to the point of excitation). This is obviously liable to affect the 

 tonus of the muscle, so that it does not always yield satisfactory 

 results ; in other cases, however, the experiment is perfectly 

 successful. 



VI. POLAR EXCITATION OF OTHER NERVES AND SPECIFIC 



NERVE-ENDINGS 



Bonders (44) has established polar action in the inhibitory 

 fibres of the cardiac vagus according to Pnuger's law, by graphically 

 recording the heart-beats. With adequate closure or opening of a 

 constant current there is plainly seen, after a short latent period, 

 to be lengthening of the succeeding, and especially of the next two 

 pulsations ; and with increasing strength of current the order is 

 as follows ascending make, descending make, descending break, 

 ascending break. The effects of ascending make and descending 

 break soon reach a maximum, after which they decline, and fail 

 even with strong currents, thus corresponding exactly with the 

 law of contraction. 



In view of the sluggishness of most smooth muscles, and their 

 consequent inability to react to a single brief stimulus, we should 

 a priori expect the manifestations of the law of polar excitation to 

 fail altogether, or at most to appear exceptionally, with indirect 

 excitation. Thus a single closure or opening of the constant 

 current produces no effect on the cervical sympathetic, while 

 repeated closure and opening result in unmistakable constriction of 

 the vessels of the ear (in rabbit). On the other hand, Pfliiger's 

 law is easily demonstrated on the comparatively quickly reacting 

 muscles of the sphincter iridis (of cat). So, too, on the mantle- 

 nerves of Eledone (v. Uexkiill, 45). The closure and opening of 

 medium currents cause contractions with both ascending and 

 descending direction. Closure of a strong descending current 

 gives tetanus throughout the period of closure, but has no effect 

 with ascending direction ; opening of the circuit in this case pro- 

 duces prolonged opening tetanus. The descending closure tetanus 

 is often rhythmical. 



