CHAPTER X 



THE NATURAL CURRENT AND ITS VARIATIONS 



Natural current in anisotropic organ from the less to the more excitable External 

 stimulus induces responsive current in opposite direction Increase of internal 

 energy induces positive, and decrease negative, variation of natural current 

 Effect on natural current of variation of temperature Effect of sudden 

 variation Variation of natural current by chemical agents, referred to 

 physiological reaction Agents which render tissue excitable, induce the 

 positive, and those which cause excitation, the negative variation Action of 

 hydrochloric acid Action of Na 2 CO 3 Effect modified by strength of dose 

 Effect of CO 2 and of alcohol vapour Natural current and its variations 

 Extreme unreliability of negative variation so-called as test of excitatory 

 reaction Reversal of natural current by excessive cold or by stimulation 

 Reversal of normal response under sub-tonicity or fatigue. 



WE have seen that when the pulvinus of Mimosa is excited 

 by an external stimulus, there is a relatively greater expulsion 

 of water from the more excitable lower half, with a con- 

 comitant greater contraction. Conversely, the lower half of 

 the pulvinus is capable of absorbing more water, and of 

 expanding to a greater extent, than the upper. Increased 

 internal energy, in contrast to the action of external stimulus, 

 has the effect of causing a greater expansion of the lower 

 half of the pulvinus, and thus raising the leaf. This we saw 

 exemplified when the plant was subjected to a gradually 

 rising temperature, so as to increase its internal energy, its 

 leaves being thereby made to show increased erection (p. 72). 

 Hence the more excitable tissue in the pulvinus of Mimosa 

 is characterised, both by greater power of absorption and by 

 greater emission of energy, according to circumstances. In 

 this we see a close analogy to the action of inorganic bodies, 

 in which also we find the greatest power of emission to be 

 associated with a correspondingly great power of absorption of 

 energy. 



