NERVE 68 1 



lation. 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-millionth part of 



the minimum work necessary for mechanical stimulation. Again, 

 with electrical stimulation (closure of a voltaic current, or condenser 



discharges) it has been shown that an amount of work equal to -^ 



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. 636). 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 experiment 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. 719) as the measure of the nervous excitation. Sum- 

 mation of stimuli, superposition of contractions, and complete 

 tetanus, are caused by stimulating a muscle through its nerve, just 

 as by stimulating the muscle itself (p. 655). 



Excitability of Nerve. It has usually been stated that the 

 excitability 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 strength- 

 ening 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 are taken to keep the current intensity the same at the 

 various temperatures compared, it is found that cooling of a 

 (frog's) nerve, even to 5 C, increases the excitability for currents 

 of long duration (several hundredths of a second). It has, indeed, 



