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256 PHYSIOLOGY CHAP. 



In order to account for the complex of phenomena comprised 

 under the term "animal electricity," Du Bois-Eeymond pro- 

 pounded his molecular theory, according to which the nerve 

 contains a large number of peripolar electrical molecules, arranged 

 in regular order. But this theory, which now has only historical 

 interest, seems neither acceptable nor necessary after the rigorous 

 criticism of data which led Hermann to formulate the ((Iteration 

 theory, accepted by most physiologists. Du Bois-Eeyrnond failed 

 to show how his molecular theory could account for the intimate 

 mechanism of the conduction of excitation along the nerve. 



To-day it is almost universally admitted (supra, p. 200) that 

 the conduction of excitation is caused by a physico-chemical pro- 

 cess in the living matter of the axis-cylinder, which is propagated 

 from one segment to another like a spark, one segment or portion 

 of the fibre being excited by the next, as though the state of the 

 active portion acted as a stimulus upon the inactive. 



This schematic conception of neural conductivity obviously 

 connotes the theory that the two physiological attributes of nerve, 

 excitability and conductivity, are fundamentally only different 

 expressions of one single property. For if we assume the con- 

 duction incited by an external stimulus to be due to the fact that 

 the active state of the excited segment acts as an internal stimulus 

 for the next segment, then conductivity must obviously be con- 

 ceived as a particular form of excitability, and the existence of 

 the first is not admissible without the second. 



Tbis theory has been opposed by a wbole series of facts, which 

 seem to show that under certain conditions conductivity can be 

 diminished, or even abolished, without perceptible injury to 

 excitability, and vice versa (see p. 229 for the local influence of 

 anaesthetics upon nerve) ; and that under many other experimental 

 conditions the rise or fall of the two properties are not parallel, 

 (see p. 245, katelectrotonic and anelectrotonic alterations of 

 excitability and conductivity). Nor can we absolutely deny the 

 contention of van Deen, Schitf, and others, that the central nervous 

 system contains fibres endowed with perfect physiological con- 

 ductivity (aesthesodic and kinesodic fibres), which are entirely 

 devoid of excitability to any artificial stimuli. But even if well 

 established these facts do not prove that excitability and con- 

 ductivity cannot co-exist ; at most they show that different nerve 

 fibres, or the same fibres under different experimental conditions, 

 present great variations of susceptibility to various stimuli and 

 their respective modes of action. It is quite probable, as Hermann 

 pointed out, that adequate internal stimuli normally find more 

 favourable conditions of excitation and conduction in nerve than 

 do the artificial external stimuli which are foreign to physiological 

 life. In this connection the work of Gotch and Macdonald on 

 the influence of temperature upon the excitability and con- 



