EXCITATION AND INHIBITION 379 



a volume easily visible under the microscope and, if filled with oxygen as 

 atmospheric pressure, would contain 3 x 10 9 molecules, since 1 cm. contains 

 4-5 x 10 1(i . This result makes it impossible to suppose that any chemical 

 process resulting in an irreversible loss of energy, such as an oxidation, can be 

 involved, and indicates that a reversible physico-chemical one of some kind is to 

 be looked for. Moreover, it makes it a matter of necessity to examine with 

 care statements that have been made as to the production of carbon dioxide by 

 nerve in activity. 



Waller (1896) noticed that the action of small amounts of carbon dioxide on nerve was to 

 increase the extent of the electrical changes and that tetanisation had the same effect. The 

 natural conclusion was that carbon dioxide was produced by the nerve in its own activity. It 

 will be clear, however, that it is quite possible that the effect of prolonged electrical excitation 

 may have nothing to do with the effect of carbon dioxide, but perhaps be a consequence of 

 some direct effect of the current, electrolytic or other effect. A recent paper by Tashiro 

 (1913, 1) claims to have proved by a direct method that carbon dioxide is formed by nerve 

 fibres as a product of their activity. A very delicate method was used for the estimation of 

 carbon dioxide (1913, 2), dependent on the formation of a film of barium carbonate on the 

 surface of a drop of barium hydroxide solution. The quantities found were, of course, very 

 small, and the possibility that they were merely dissolved in the tissue is not satisfactorily 

 excluded. Small amounts were given off very slowly by the resting nerve. The extra pro- 

 duction on electrical excitation would easily be accounted for by the heat resulting from the 

 passage of the current. It is to be remembered that a nerve trunk consists not only of nerve 

 fibres themselves, but of connective tissue in which 

 are living cells, which would contain carbon dioxide 

 from their own respiration, so that, although the pro- 

 duction of carbon dioxide on their part might not be 

 increased by electrical excitation, that dissolved in 

 them would be partly given off in consequence of the 

 heat produced by the exciting current itself. We 

 note also that there was a regular falling off in the 

 output of carbon dioxide by resting nerve, even dur- 

 ing the time in which there is every reason to sup- 

 pose that the excitability was undiminished. Killing 

 the nerves by the action of steam abolished their 

 power of giving off carbon dioxide, both resting and YIG. 101. DIPHASIC CURRENT OF ACTION 

 on excitation ; but this does not show that it was a IN OLFACTORY NERVE OF PIKE, AS 



vital phenomenon, since carbon dioxide dissolved in SHOWN BY THE CAPILLARY ELECTRO- 



the tissues would naturally be driven off before the METER. 



observation. The numerical data given throw doubt 



on the origin of carbon dioxide from the metabolism ^"^f n of w j* a 8 ; " gle break mductlon shock 

 of the nerve itself, since they show that it is higher Tim ^ ;" '^ ^"nd. 



than that of an equal weight ot muscle. Until (After Garten.) 



experiments are made in which the source of error 

 referred to is excluded, I fear that we cannot accept 



the conclusions of Tashiro. The source of the effect in Waller's experiments may also have 

 been from carbon dioxide dissolved in non-nervous tissue. 



We are thus limited to the electrical change. Fortunately, this can be 

 measured with accuracy and its time course determined. For the present, we 

 will omit discussion as to its cause, and limit ourselves to the fact that a spot 

 in a state of excitation behaves as if electrically negative to a spot on the nerve 

 at rest ; that is, if the two points are connected to a galvanometer, a current 

 flows through the instrument from the resting to the excited spot, as if the former 

 corresponded to the copper of a Daniell battery and the latter to the zinc. We 

 find that the electrical change set up at one point by a momentary stimulus lasts 

 only for a short time at this point and passes along the nerve, making each point 

 in turn electro-negative to the rest. Suppose that two electrodes lie on the nerve 

 at different points and that a stimulus is applied near the one electrode ; the 

 electrical response will consist of a current in one direction as this electrode 

 becomes negative to the more distant one, followed by a current in the opposite 

 direction as the wave, having left the first electrode, arrives at the second, 

 making it negative to the other. If we call the electrodes A and B, we have, 

 first, A negative, B positive, then A positive, B negative. This form of 

 electrical response is called "diphasic" (see Fig. 101). If electrode B 

 is on a spot which has been killed, the wave of negativity, as it is com- 

 monly called, disappears in the killed area, so that the only electrical effect 

 seen is that due to the becoming negative of spot A and consists, therefore, 



