VIM CONDUCTIVITY AND EXCITABILITY OF NERVE 93 



excitable near the point where they leave the spinal cord than 

 lower down, and at this last point again than at a still lower place, 

 etc." He concluded that "greater force must be employed to 

 evoke a twitch, in proportion as the seat of excitation is further 

 from the spinal origin of the nerve, or (which is the same thing) 

 nearer to its insertion in the muscle." He found, moreover, in the 

 course of his experiments that certain points in the nerve " are 

 much more excitable than others lying both above and below 

 them, while others again are characterised by great inexcitability." 

 These last he termed " nodal points." The excitation of one point 

 of a nerve at a given strength of current will often discharge a 

 perceptible twitch, while a point 1 mm. off, with the same strength 

 of current, gives no sign of contraction. One point especially 



FIG. 163. 



noticeable in this connection lies in the middle third of the thigh, 

 where the nerve gives off a large branch. Another such is 

 commonly found near the starting-point of the motor roots. 

 Pf! tiger (43) subsequently summed up all these facts in the dictum 

 that " the same stimulus, applied in succession to two different 

 points of the nerve, does not excite the muscle in the same degree ; 

 that excitation is the more effective which is applied at the point 

 most distal to the muscle." The highest tract alone, next to the 

 section, was relatively less excitable. According to Pfliiger, the 

 curve of excitability drawn from the nerve as abscissa would, 

 therefore, resemble Tig. 163. There is a depression at the point 

 where the branch is given off to the thigh. It is questionable 

 whether the ordiuates of this curve can really be regarded 

 as a direct measure of excitability. It is evident that they 

 express only relative magnitudes of stimulation -effects at the 

 different points of the nerve, two explanations of the diminution 

 towards the periphery being conceivable. Either the excitability 

 of the nerve is greater in proportion as the point stimulated 

 lies nearer the central organ, or the same stimulus calls out 



