54° 



NER VE. 



Finally the positive after effect or demarcation increment seems, 

 in many cases, to be similarly changed into the more usual negative 

 after effect, i.e. demarcation decrement. 



Precisely similar changes are caused by C0 2 gas, the most modified 

 effects being transformed into less modified, and these latter into those 

 of the freshly excised nerve. Examples of both these sets of conditions 

 are given in Fig. 278. 



It is thus clear that a close similarity exists between the action 

 of previous activity and that of CO, on the electromotive phenomena 

 of stale or modified nerves, and this is the foundation for Waller's 

 view, that the changes following previous activity are caused by 

 the development of C0 2 in the nervous substance, in consequence of the 

 metabolism accompanying such activity. 



From these experiments Waller infers that the electrical responses of nerve 

 are in reality the algebraic sum of changes of opposite sign. In the freshly 

 excised nerve those indicating negativity (i.e. diminution of the resting 

 difference) are far the. larger, in excised nerves kept for some hours those 

 indicating negativity have declined, and positivity may be so pronounced as to 

 swamp the feeble negative changes. The action of prolonged excitation and 

 of CO., gas in sweeping away the positive effects, may be due either to their 

 diminishing the molecular change which is the basis of these, or more probably 

 to their increasing the change which underlies the negative effects. If the 

 galvanometric effects are to be regarded as a faithful indication of the excita- 

 tory processes, then their twofold character must be interpreted as meaning 

 that, in nerve, two excitatory changes of opposite sign are transmitted along an 

 excited nerve. 



It must be confessed that the capillary electrometer records previously 

 described do not support this view, since, whatever the character of the after 

 effect, one change only occurs in kept nerves as the initial effect, i.e. relative 

 negativity. The results obtained by Waller may be explained on the view 

 that the positive effects are all effects due to the development of relative 

 negativity of the tissue under the distal contact. The method used by him, 

 unless extended to comprise rheotome observations, is incapable of giving 

 precise information as to the seat of the changes producing the positive effects. 

 This is of essential importance in connection with his conceptions of the ex- 

 citatory processes, and until we have evidence that the positive change occurs 

 under the proximal contact, the views of Waller, based upon the supposition 

 that it does occur there, cannot be regarded as well founded. 



Electromotive Changes in Nerve associated with the 

 Passage of Electrical Currents. 



Polarisation effects— Electrolysis— External and internal polari- 

 sation.-— The passage of a galvanic current through a moist conductor is 

 attended by well-known electrolytic phenomena, which subside more or less 

 rapidly on its cessation. The phenomena in question are due to the production 

 of electrolytic changes attaining their maximal development at the points 

 where the" current enters and leaves the moist conductor, i.e. at the metallic 

 contacts of the battery circuit with this. The electrolytic changes produced 

 by strong currents are such that bubbles of oxygen emerge at the anode, 

 bubbles of hydrogen at the cathode ; with feebler currents no obvious evolution 

 of gas takes place, and the products of electrolysis, or electrolytic " ions," are 

 less evident. Evidence of the formation of ions in these instances is 

 obtained, when the moist conductor contains neutral salts, by acidity^ of 

 the liquid at the anode and alkalinity at the cathode, and an appropriate 



