RAPIDITY OF CONDUCTION OF THE STIMULUS IN NERVES. 667 



traction that is due really to closing of the nerve-current branch previously di- 

 verted in consequence of secondary closure (through the electrodes) . 



If a motor nerve is acted on simultaneously by two induced currents the 

 following two results are possible. One induced current may be so feeble 

 that the nerve is not irritated by it to the point of contraction, while the other 

 induces only a feeble contraction. In this event the inframinimal current plays 

 the part of a feeble constant current, and the size of the contraction depends only 

 upon whether the effective irritating current is applied near the anode or the 

 kathode of the inframinimal current. If, however, two stimulating currents of 

 unequal strength, separated by a considerable distance from each other, in order 

 to exclude electrotonic effects, are applied to a nerve, and each of them alone 

 is effective, the same result occurs as if the stronger stimulus alone were applied. 

 The feebler wave of stimulus is lost entirely in the stronger. 



If in man a nerve is compressed and the affected portion of the body is ren- 

 dered anemic by compression of its arteries, the opening contractions soon pre- 

 dominate greatly and kathodal opening contraction in greater degree than anodal 

 opening contraction compression-reaction of R. Geigl. 



RAPIDITY OF CONDUCTION OF THE STIMULUS IN NERVES. 



If a motor nerve is stimulated at its central extremity the impulse 

 is propagated like a wave-movement through the course of the nerve to 

 the muscle with great rapidity, which in the case of the sciatic nerve 

 of the frog is equal to 27.25 meters in a second, for the motor nerves, 

 and in that of man, 33.9 meters. 



The rapidity of conduction is apparently less in the visceral nerves; for ex- 

 ample, 8.2 meters in the pharyngeal fibers of the vagus. Fredericq and van de 

 Velde found the rate in the motor nerves of the lobster to be 6 meters. 



The propagation-rapidity of the wave of excitation is susceptible to various 

 influences. It is retarded by cold and also by considerable heat applied to the 

 nerve, by curare, and by anelectrotonus, while it is increased by katelectrotonus 

 in the freely exposed nerve. It varies with the length of the conducting portion 

 of nerve, but it increases with the strength of the stimulus, though not at first. 

 The power of conduction is diminished in the anelectrotonic portion. 



Method. v. Helmholtz determined the velocity of propagation of the 

 impulse for the motor nerves of the frog according to the method of Pouillet, 

 which is based on the fact that the needle of the galvanometer is deflected by a 

 constant current of short duration. The degree of deflection is proportional to 

 the duration and the strength of the current, which is known in this instance. 

 The method itself is so applied that the time-measuring current is closed at the 

 moment that the nerve is irritated, and it is again opened when the muscle 

 contracts. If, now, the nerve is irritated first at the central extremity and 

 then close to its entrance into the muscle, the time between the beginning of 

 the stimulation and the contraction will in the latter event be shorter, and 

 therefore the deflection of the galvanometer will be less, than in the first case, 

 as the stimulus must pass through the entire nerve to the muscle. The difference 

 between the two periods of time is the propagation-time for the stimulus in the 

 portion of nerve examined. 



Fig. 235 is a diagrammatic representation of the arrangement of the experi- 

 ment. The galvanometer G is introduced into the (still open) circuit a, b, c, d, 

 e, f, g, h, yielding the time-measuring current. Closure is effected by depressing 

 the lever S, the platinum plate c of the pivoted arrangement W being tilted down 

 by d. At once, with the occurrence of closure the magnetic needle is deflected 

 and the extent of the deflection is noted. At the same moment that the current 

 between c and d is closed the primary circuit of the induction-apparatus, k, i, p, 

 O. m, 1, is opened by raising the extremity of the pivoted arrangement at i. By 

 this means an opening current is induced in the induction-spiral R, which stimu- 

 lates the nerve of the suspended frog's leg at n. The impulse is propagated 

 through the nerve to the muscle (M) ; the latter contracts as soon as the impulse 

 reaches it, and, by raising the lever H, which can be rotated about x, opens the 

 time-measuring current by means of the double contact e and f. At the moment 

 of opening, the further deflection of the magnetic needle ceases. The contact at 

 f consists of a mercurial dome drawn out to a thread. If, after the contraction 



