CIRCUMSTANCES AFFECTING NERVE EXCITABIIITY. 477 



probable that nerves differ as to the possession of relative indefatiga- 

 bility and susceptibility to stimulation by rapidly recurring excitations. 

 The rapid failure of non-medullated nerves, when removed from their 

 connections, appears to indicate that they do not possess such properties 

 to the same extent as medullated motor nerves. 



Circumstances affecting Nerve Excitability and 



Conductivity. 



Maintenance of the circulation. — It is common knowledge that 

 the blood supply is essential for the maintenance in their normal condi- 

 tion of the living structures of higher animals, and that the failure of 

 vital activity in consequence of its cessation is one which is more rapidly 

 produced in warm- than in cold-blooded animals. Thus the blood can be 

 replaced in the frog by water containing suitable proportions of NaCl, 

 KC1, CaC0 3 , etc., and the animal kept alive in an apparently normal state 

 for many days. A similar variation, as regards dependence upon blood 

 supply, is observed in the different structures of any one animal, and in 

 the different parts of the nervous system. It is well known that the 

 central portions of this system are the first to fail in consequence of 

 the cessation of the circulation, and that the cerebral cortex is more 

 rapidly impaired than the bulbo-spinal system, whilst excised nerve 

 trunks retain their excitability for a very considerable time. Such 

 retention implies the possession by the fibres of nerve trunks of pro- 

 perties which enable them to resist the gradual invasion of those 

 changes that bring about their death. This resistance depends very 

 largely upon the general condition of the animal previous to the time 

 at which the nerve is excised. Thus frogs kept in captivity during the 

 summer become anaemic, and their nerves more rapidly fail on exposure 

 than those of winter frogs. 



It is to be expected, in view of the facts as to indefatigability, etc., 

 brought forward in the preceding chapter, that nerve fibres should offer 

 a very considerable resistance to functional failure, in consequence of 

 the arrest of the circulation, their metabolism being such as to require 

 but little assistance from extraneous sources. 



If injurious effects of other kinds, such as loss of water through 

 drying, temperature changes, etc., are avoided, the medullated fibres of 

 a nerve trunk maintain their excitability for many hours, and, in the 

 case of the frog, for days after the circulation has ceased. 



Ritter-Valli law. — In the medullated fibres of the frog's sciatic, the 

 failure is one which is gradual in its development, and is more rapidly 

 produced in the central than the distal portions of the nerve. This is 

 enunciated in the Eitter-Valli law, which states that the susceptibility 

 of the nervous system to excitation, as indicated by the muscle response, 

 fails first in the central portions, then in the peripheral nerve trunks, 

 and, last, in the terminal arborisations within the muscle. 1 



The experimental proof of these statements is, as regards a nerve 

 trunk, complicated by the alterations produced by nerve-section ; this 

 always produces a very considerable rise in excitability, due to the 

 electromotive changes caused in the neighbourhood of the section. 

 These changes are such that a persistent current (demarcation current) 

 flows through the nerve fibres from the injured to the uninjured portions 



1 Vide Budge, " Lehrbuch. d. Physiologic," Leipzig, 1862, S. 665. 



