458 NERVE. 



this highly improbable premise were granted, would the deduction as to trans- 

 mission in the lingual be sound, since this is known to possess efferent fibres 

 derived from the chorda tympani. Langley 1 has recently recorded experiments 

 on the rabbit, which seem to show that the central end of the cervical vagus can 

 establish physiological connection Avith the cells of the superior ganglion of the 

 cervical sympathetic, if the cut cephalic ends of these two nerves are brought 

 into contiguity and left for from seventy-three to 123 days. 



The conductivity of any nerve fibre is influenced by a large number of 

 conditions ; these are especially changes in its environment, and will be 

 considered in connection with alterations in excitability due to the same 

 causes. One influence only will be referred to now, namely, that due to 

 the intensity of the excitatory state. It was observed by Helmholtz, in his 

 experiments on the rate of propagation along the nerves of the arm in 

 man, that the excitatory state evoked in nerve by an intense stimulus 

 was propagated more rapidly than that caused by a weaker one. This 

 result has been confirmed in the motor nerve of the frog by Vintschgau, 2 

 and in the non-medullated nerves of Anodon by A. Fick. 3 



It is, however, questionable whether this result is true for all forms 

 of stimulation, mechanical as well as electrical ; the employment of the 

 latter as the means of starting the excitatory state is open to the 

 criticism that, when intense, it is difficult to localise it. Since the 

 precise localisation of the starting point is essential for experiments 

 upon the rate of propagation, any uncertainty as to its being strictly 

 limited must render the results dubious. The apparent decrease in the 

 period between nerve excitation and muscle response, when the nerve 

 is stimulated with stronger induced currents, may, in many cases, be due 

 to the spread of such currents along the nerve, and the consequent 

 stimulation of portions which lie between the electrodes and the 

 responding muscles. 



The essential facts of nerve transmission may be summed up as 

 dependent upon the successive excitation of the various contiguous por- 

 tions of the nerve fibre. Two conditions must be assumed to determine 

 the realisation of this succession, the potency of the exciting cause, 

 and the degree of susceptibility of the tissue which has to be aroused. 

 Our present knowledge leads us to regard the exciting cause as an agent 

 able to produce a particular molecular rearrangement in the nerve ; this 

 constitutes the state of excitation, and is accompanied by localised 

 electrical changes as an ascertained physical concomitant. It 

 has been suggested that these local electrical changes are themselves 

 the agents concerned in the stimulation of the quiescent portions 

 of nerve in immediate proximity, and that this process is carried on in 

 succession down to the nerve-endings, where it forms the releasing 

 agent for the muscular response. 4 It is certain that electrolytic 

 processes play a most important part in the whole process, and that 

 the intensity of any one local excitatory effect determines, other things 

 being equal, that of the effect aroused in the neighbouring portion. The 

 view is supported by other considerations, which will be alluded to 

 later. 



1 Proc. Boy. Soc. London, 1898, vol. lxii. p. 331; " Proc. Physiol. Soc," in Journ. 

 Physiol., Cambridge and London, 1899, vol. xxiv. 



2 Arch. f. d. ges. Physiol., Bonn, Bd. xxx. and xl. 



3 " Vergleich. Physiol, der irritabl. Substanzen," 1863. 



4 Kiihne, "Causation of Vital Movement," Ctoonian Lecture, 1888, Proc. Roy. Soc. 

 London. 



