784 NERVE 



than a gramme-millimetre that is, the work done by a gramme falling 

 through a distance of a millimetre, or (taking an erg as equivalent to 

 i^rro gramme-centimetre) about 100 ergs. No doubt a great part of 

 this is wasted, as a much smaller quantity of work done by a beam of 

 light on the retina or by an electrical current on an isolated nerve, both 

 of which may be supposed to act more directly on the excitable con- 

 stituents, suffices to cause stimulation. Thus, the work done by the 

 minimal, natural or specific, stimulus for the retina in the form of green 



light may be as little as g erg (S. P. Langley), or only one-ten-thousand - 



millioneth part of the minimum work necessary for mechanical stimula- 

 tion. Again, with electrical stimulation (closure of a voltaic cuirent, 

 or condenser discharges) it has been shown that an amount of work 



equal to 4 erg may be enough to cause excitation of a frog's nerve. 



This is ten thousand times as great as the minimal luminous stimulus, 

 but a million times less than the minimal mechanical stimulus. 



The laws of electrical stimulation for nerve are essentially the same 

 as those we have already discussed for muscle (p. 741). The voltaic 

 current stimulates a nerve, as it does a muscle, at closure and opening. 

 During the flow of the current, so long as its intensity remains constant, 

 there is as a rule no excitation, or at least none which is propagated 

 along the nerve, so that the muscles supplied by it remain uncontracted. 

 But under certain conditions for example, when the nerve is more 

 excitable than usual (as is the case with nerves taken from frogs which 

 have been long kept in the cold) a closing tetanus may be seen while 

 the current continues to pass through the nerve, and an opening tetanus 

 after it has ceased to flow, just as when the current is led directly 

 through the muscle. Sensory nerve-fibres, too, are stimulated by a 

 voltaic current during the whole time of flow. Induction shocks are 

 relatively more powerful stimuli for nerve than the make or break of a 

 voltaic current. The opposite, as we have seen, is true of muscle; and, 

 upon the whole, we may say that muscle is more sluggish in its response 

 to stimuli, and is excited less easily by very brief currents, than nerve 

 is. An apparent illustration of this difference is the fact that the 

 nervous excitation has no measurable latent period, while muscular 

 excitation has. But it is quite possible that, if the conditions of experi- 

 ment were as favourable in nerve as in muscle, a sensible latent period 

 might be found here too. 



In nerve as in muscle, strength of stimulus and intensity of response 

 correspond within a fairly wide range, when we take the height of the 

 muscular contraction or the amount of the negative variation (p. 824) 

 as the measure of the nervous excitation. Summation of stimuli, super- 

 position of contractions, and complete tetanus, are caused by stimulating 

 the muscle through its nerve, just as by stimulating the muscle itself 

 (p. 756). 



Excitability of Nerve. It has usually been stated that the ex- 

 citability of frog's nerve, as measured by the muscular response to 

 stimulation, is increased by rise of temperature, and diminished by 

 fall of temperature. It has, however, been shown that this increase 

 of excitability is only apparent, and due to the strengthening of 

 the current by diminution of the resistance, since the resistance 

 of all animal tissues, like that of electrolytic conductors in general, 

 diminishes as the temperature rises (Gotch). When precautions 



