470 NERVE. 



undoubted index of nerve excitation, but its absence cannot be taken 

 as convincing proof of the failure of such nerve excitation. It is con- 

 ceivable that the nerve equilibrium, although upset more potently at 

 the moment of onset and cessation, may be similarly disturbed during 

 and after the flow of a current, and indeed we have experimental 

 proofs (electrotonus) that such is the case. 



This prolonged disturbance is not termed in ordinary physiological 

 parlance an excitation simply because the attached muscle does not 

 respond on its being propagated to the motorial nerve- endings. The 

 electromotive changes, which are the index of the production of the 

 true nervous disturbance, are however present, and the disturbance 

 may under favourable conditions be so pronounced that a muscular 

 response is evoked, thus definitely proving its existence. It is well 

 known that in the case of cooled nerves, or those which have been treated 

 by chemical reagents (NaCl, etc.), either the closing or opening 

 muscular twitch may be succeeded by a prolonged tetanic contraction. 



In the case of other nerves, such as those of the adductor and 

 abductor muscles of the crayfish claw, prolonged excitatory effects, 

 evoking relaxation or augmentation of the muscular tonic contrac- 

 tion, occur during and following the closure of the current, whilst 

 voluntary muscle responds by a continuous contraction to the passage 

 of a galvanic current through its substance. The sciatic nerve of 

 the cooled frog may be persistently stimulated by the demarcation 

 current caused by its section, the excitation evoking a continuous 

 muscular contraction. 1 It has been shown that stimulation of the 

 afferent fibres in the vagus nerve of the mammal is produced during 

 the whole time of the passage of such a current, as evidenced by the 

 respiratory movements.' 2 The glandular response to excitation of the 

 glosso-pharyngeal nerve is similarly evoked, not merely at the onset, but 

 still more effectually during the flow of a galvanic current through this 

 nerve. 3 Finally, it was shown by Konig 4 that the duration of a 

 galvanic current was an important factor in determining its efficiency as 

 a nerve stimulus ; this is strikingly the case in involuntary muscle, but 

 it is easily demonstrable in cooled nerve, that for any given intensity 

 there is a minimal period of closure below which the stimulating effect 

 is decreased. Since the rise of the current intensity is practically 

 instantaneous, this fact can only be interpreted as meaning a less 

 efficient excitation, in consequence of the shortness of the period 

 of flow. 



As regards nerve, therefore, the first statement in the law of ex- 

 citation requires limitation. Nerve being of all structures that in 

 which the properties of excitability are most pronounced, is differentiated, 

 so that it is more susceptible to variations in current intensity than to 

 the maintenance of such intensity ; but the excitatory condition, though 

 produced at the onset and the cessation of the current, can be also 

 evoked, though in less degree, both during and after the current flow. 



The second portion of du Bois-Eeymond's statement is, as regards 

 the motor nerves of the frog and the mammal, of far more general 



1 Hering, SUzungsb. d. k. Akad. d. Wissensch., Wien, 1882, Bd. Ixxxv. Abth. 3, 

 S. 237; also, "Biological Memoirs," Oxford, 1887, p. 136. 



2 Langendorff and Oldag, Arch. f. d. get. Physiol., Bonn, 1894, Bd. lix. 



3 Biederraann, ibid., 1893, Bd. liv. 



4 Konig, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1870, Abth. 2. 



