NATURE OF THE NERVE IMPULSE. 113 



in reaction can be obtained by means of the usual indicators for 

 acidity. Waller has given some experiments to show that carbon 

 dioxid is produced during activity, but they are far from being con- 

 clusive. His line of argument is as follows: He has found that the 

 action current of a nerve that is being stimulated is increased by the 

 presence of very slight amounts of carbon dioxid, higher percentages 

 causing again naturally a decrease. This reaction for the presence 

 of carbon dioxid is apparently a very delicate one. When now a 

 normal nerve is stimulated its action current after some minutes 

 of tetanic stimulation is increased in the same way as would 

 happen if a little carbon dioxid was passed over it. He con- 

 siders that this temporary increase in the action current is due 

 to the formation of carbon dioxid from a functional metabolism. 

 The only significant evidence that we have of a chemical change in 

 the fiber during activity is found in two facts already mentioned: 

 one is the discovery that oxygen is requisite for normal conduction. 

 A nerve placed in an atmosphere free from oxygen loses its irrita- 

 bility, and regains it again quickly upon the admission of oxygen. 

 The other is found in the statement of Bethe, that when a nerve is 

 stimulated a definite change in the staining property of the neuro- 

 fibrils may be noted (see p. 102). At present we must admit, 

 therefore, that so far as the nerve fiber is concerned, we have no 

 positive proof of a functional metabolism. This negative state 

 of our knowledge, considering the difficulties involved in obtaining 

 proofs, hardly warrants a positive denial of the existence of such 

 a metabolism. All tissues whose chemistry can be studied show 

 a metabolism during functional activity, and, reasoning from 

 analogy, it seems probable that the same fact will eventually be 

 demonstrated for the axis cylinder. 



Views as to the Nature of the Nerve Impulse. The older con- 

 ceptions of the nerve principle, while they varied in detail, were 

 based upon the general idea that the nervous system contains a 

 matter of a finer sort than that visible to our senses. This matter 

 was pictured at first as a spirit (animal spirits), and later as a ma- 

 terial comparable to the luminiferous ether or to electricity. Since 

 the discovery that the nerve impulse travels with a relatively slow 

 velocity and is accompanied by a demonstrable change in the electrical 

 condition of the nerve, two main views regarding its nature have been 

 entertained. Many physiologists conceive the nerve impulse 

 as a progressive wave of chemical change which is started at one 

 end by the stimulus and is then self-propagated along the fiber. 

 The conception in general is represented by the transmission of a 

 spark along a line of gunpowder. The flame applied at one end 

 causes an explosive chemical change, which is then propagated 

 from point to point. The analogy is obviously very incomplete, 

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