viii CONDUCTIVITY AND EXCITABILITY OF NERVE 67 



conductivity. It is plain that if the time occupied by the 

 passage of excitation through the ganglion is perceptibly longer 

 than the known time of conductivity through an equal tract 

 of normal nerve-fibre, there must be an interruption of some 

 kind. The only histological elements in the ganglia which can 

 present such an interruption are, however, the nerve-cells. The 

 relations are far simpler here than in the central nervous 

 system, where also the time occupied by the transmission of exci- 

 tation has been taken as a proof of the existence of special elements 

 interpolated along the course of the simple conducting paths; there, 

 however, the delay in transmission must be referred not merely to 

 nerve-cells, but also to the nervous network which is possibly 

 present. Exner, who undertook to determine the time in 

 which the centripetal wave of excitation traverses the frog's 

 spinal ganglia, employed Bernstein's rheotome to measure (on 

 the sciatic, ganglion, and posterior root) the interval between the 

 excitation of the sciatic and the arrival of the negative variation 

 in the fibres of the posterior root, as led off to the galvanometer. 

 He obtained figures below those quoted by Bernstein for the 

 rapidity in normal peripheral nerve, and concluded that con- 

 ductivity was not blocked at the ganglion. The rheotome method 

 is, however, open to many objections. Wundt (29) had pre- 

 viously tried to determine the point by testing the influence of 

 the spinal ganglia upon reflex excitability. Curves of twitches 

 from the muscles of one leg (in the frog), obtained by alternately 

 exciting the opposite sciatic trunk and a posterior root on the 

 central side of the ganglion (between the ganglion and the spinal 

 cord), invariably gave a marked difference of latent period, corre- 

 sponding with a delay in conductivity at the ganglion. Gad (30) 

 repeated the same experiments on the jugular ganglion of the 

 rabbit's vagus. The reaction to be determined was in this case 

 the reflex modification of respiratory movements, by excitation of 

 the vagus. The sole variable was the point, alternately central 

 and peripheral to the ganglion, at which the stimulus was 

 applied. The respiratory movements were graphically recorded 

 by the usual method, and in order to ensure uniformity of 

 external conditions at the centre during the respective tests, 

 apncea was induced, or the stimuli were carefully regulated for 

 the same phase of respiration. The reaction-time yielded by 

 these experiments was 



