224 PHYSIOLOGY CHAP. 



but also during the passage of the current, a continuous sensation 

 which is the subjective sign of an excited state of the nerve. 



It should be noted that besides the influence of the ascend- 

 ing or descending direction, to which we shall refer below, the 

 current is most efficacious when passed longitudinally through the 

 nerve, least when passed transversely across it (Galvani, Albrecht). 

 Finally, the exciting action increases with extension of the intra- 

 polar length (Pfaff, v. Humboldt). 



VI. The efficiency of external stimuli varies in the first place 

 with the excitability of the nerve, which differs very much not 

 only in different classes of animals, but also in different nerves of 

 the same animal, in different fibres of the same nerve, and, accord- 

 ing to some investigators, even in different parts of the same fibre. 

 Kitter and Eollett were the first to note that on exciting a frog's 

 sciatic with a current of minimum intensity the abductors and 

 flexors of the foot i.e. the muscles innervated by the peroneal 

 nerve were thrown into contraction ; while the adductors and 

 extensors i.e. the muscles innervated by the branches of the tibial 

 were only excited by stronger currents. This same holds good 

 for the flexor and extensor nerves of the toad and rabbit, and can 

 be shown not only with electrical but also with mechanical and 

 chemical stimuli. In the frog's vago-sympathetic trunk the 

 inhibitory fibres are excited by weaker currents than the acceler- 

 ators. Excitation of the nerve of the crab's claw with a very 

 weak current (see p. 35) causes the abductor of the claw to 

 contract ; with stronger currents this muscle relaxes and the 

 adductor contracts. Weak currents usually suffice to excite 

 nerves, but the nerve of the electrical organ of Malaptcrurus is 

 excitable to strong currents only, and is almost inexcitable to 

 chemical stimuli. Probably there are no two nerves in the same 

 animal with identically the same degree of excitability. 



At first sight the degree of excitability in different parts of the 

 same nerve appears to vary. If a motor nerve, e.g. the frog's 

 sciatic recently divided from the spinal cord, is excited at different 

 points nearer to or farther from the muscle, the reaction of the 

 muscle is seen to be more vigorous in proportion as the stimulation 

 is more remote. Pfliiger explained this fact (first observed by 

 Budge) on the hypothesis that the nervous excitation produced by 

 the stimulus increased like an avalanche on its way to the muscle. 

 But this interpretation was at once disputed by Heidenhain, and 

 subsequently by Fleischl, Griitzner, Tigerstedt, and others. The 

 phenomenon must be due to the increase of excitability caused in 

 the upper part of the sciatic by the injury due to the section. When 

 the nerve remains as far as possible under normal conditions, it is 

 found to be equally excitable in its different parts to chemical 

 (v. Fleischl) and mechanical stimuli (Tigerstedt). The excitatory 

 impulse is more probably weakened than reinforced during its 



