x ELECTROMOTIVE ACTION IX XERVE 235 



with the nearest accessible point of the longitudinal surface, 

 but also when the cut end drops upon that of a second 

 nerve. In the last case, however, contraction occurs only when 

 the two cut ends are not in the same line, but the one nerve 

 falls longways upon the other, so that the two ends come 

 together. There is thus mutual reaction of the two demarca- 

 tion currents, since they flow in the same direction through the 

 circuit formed by the two cut ends. (Hering previously per- 

 formed the same experiments successfully with two curarised 

 frogs' muscles sartorius). " The fact that sufficiently vigorous 

 nerves fall into persistent excitation when their own current is 

 short-circuited externally, leads us to conjecture that the above- 

 mentioned tetanic excitation, as seen in cooled frogs after dividing 

 the nerve of the leg, or the sciatic plexus, may also be caused 

 solely by the current incident upon such division " ; since the 

 sheaths of the single fibres, and the common neural integu- 

 ment, alike provide an internal path for the individual currents 

 of the fibres. 



The above facts refer to motor frogs' nerves only. It was, 

 however, shown by Knoll (13) that centripetal nerves of warm- 

 blooded animals can also, under certain conditions, be excited by 

 their own currents. These experiments refer exclusively to the 

 cervical vagus of dog and rabbit, and more particularly again to 

 the central portion, which is in connection with the respiratory 

 centre. The mere exposure of these nerves, especially where 

 they are injured, causes protracted expiration, or even a brief 

 expiratory pause in respiration, in the rabbit ; while similar 

 effects of longer duration appear with great regularity on raising 

 a cervical vagus, ligatured at the thoracic end and isolated, from 

 the wound, or lifting it out of an indifferent conducting fluid, 

 especially when the nerve has previously been divided peripher- 

 ally to the ligature (cf. Langendorff, 13). A more or less pro- 

 longed expiratory pause is again produced on dropping the 

 vagus on the wound, or moistening it with a conducting fluid 

 (0*6 / Q NaCI). Since it can be shown in all these cases 

 that neither mechanical, thermal, nor chemical stimuli come into 

 play, and since " all the factors known to produce a nerve current 

 react favourably upon the experiment as described above,"- 

 respiration further remaining unaltered " when uniformity of 

 other conditions has prevented the setting-up or reinforcement of 



