106 ELECTRO-PHYSIOLOCV CHAP. 



even in functionally analogous end -organs, e.g. striated and 

 smooth muscle, any comparison of the excitability of nerves con- 

 nected with dissimilar terminal organs is fundamentally impossible. 

 This is most plainly seen when the conditions for the discharge 

 of refle-x muscular contractions are compared with those for the 

 direct excitation of motor nerves. The great differences apparent 

 in the two cases can hardly be referred to specific differences in 

 the nerve-fibres, but derive much rather from the inherent pro- 

 perties of the nerve-cells, as described above. 



The most striking among many facts is that a single short 

 stimulus, whether mechanical or electrical, inevitably discharges 

 a twitch when the motor nerve of a striated muscle is stimulated 

 directly, but by no means as certainly in the case of reflex 

 excitation. Here, indeed, it is the rule that a single brief stimulus 

 acts only (if at all) at very high intensities. That the cause for 

 this lies not so much in special properties of the centripetal 

 nerve-fibres as in dissimilar relations of excitability in the central 

 reflex organs (nerve-cells) may be presumed from the above 

 observations on conductivity of excitation in cells and fibres. 

 We have seen that nerve-cells present a certain resistance to the 

 conduction of the excitatory process, and thus to excitation itself, 

 expressed on the one hand by a more or less conspicuous delay in 

 transmission, on the other by the greater susceptibility of the sub- 

 stance of the ganglion to short impacts of stimulation. At the 

 same time, while the great sensitiveness of the nerve-cells to any 

 kind of injury is very striking, it must be observed that in regard 

 to excitability they resemble the less excitable, sluggish, smooth 

 muscles rather than the quickly-reacting striated muscles. We 

 shall see later how much the excitation of the more sluggish 

 excitable tissues depends upon duration of stimulus, the most 

 striking proof of which is perhaps the fact that the same induc- 

 tion shock which inevitably produces a twitch of the striated 

 muscle when applied to its nerve, evokes no perceptible con- 

 traction of smooth muscles when sent into their nerve-fibres, and 

 is as little able to excite a reflex twitch from the former. As 

 regards the first, Langendorff (60) has shown that on exciting 

 the cervical sympathetic with single induction shocks there is no 

 perceptible change in the width of the pupil, whereas repeated 

 shocks become effective by " summation." With increasing 

 strength, however, Muhlert (61) was frequently able, even with 



