480 NERVOUS SYSTEM 



this kind, are extremes of heat and cold. If an exposed nerve is cauter- 

 ized, a vigorous muscular contraction follows. The same effect, though 

 less marked, may be produced by the sudden application of intense cold. 

 Among chemical reagents, there are some which excite the nerves and 

 others which produce no effect ; but these are not important from 

 a physiological point of view, except common salt, which is sometimes 

 used when it is desired to produce tetanic action. Mechanical stimula- 

 tion and the action of certain chemicals are capable of exciting the 

 nerves ; but when their action goes so far as to disorganize the fibres, 

 the conducting power of these fibres is lost. While, however, stimula- 

 tion of the nerve above the point of such injury has no effect, stimulation 

 between this point and the muscles is still followed by contraction. 



The excitability of a nerve is not to be confounded with its conduc- 

 tivity. A nerve is excitable when its stimulation excites muscular action, 

 the impulse being conducted to the muscle. Acting thus as a conductor, 

 the nerve is said to possess conductivity. It is possible to destroy for 

 a time the excitability of a restricted portion of a nerve without affect- 

 ing its conductivity ; for when a stimulus is applied to the nerve above 

 this portion, it is followed by muscular contraction, the impulse being 

 conducted through the portion that has been rendered inexcitable. 

 Some parts of the nervous system, indeed, normally are inexcitable 

 under direct stimulation, but nevertheless act as conductors. 



The most convenient method of exciting the nerves in physiological 

 experiments is by means of electricity. This may be employed without 

 disorganizing the nerve-tissue, and it consequently admits of extended 

 and repeated application. The action of electricity, however, with the 

 methods of preparing the nerves and muscles for experimentation, will 

 be considered under a separate head. 



Rapidity of Nervous Conduction. The first accurate estimates of 

 the rapidity of nervous conduction were made by Helmholtz, in 1850, 

 and were applied to the motor nerves of the frog. These estimates 

 were arrived at by an application of the graphic method, which was 

 afterward considerably extended and improved by Marey. The process 

 employed by Marey, which is essentially the same as that used in more 

 recent investigations, is the following : 



To mark small fractions of a second, a tuning-fork vibrating at a 

 known rate (five hundred times in a second) is so arranged that a point 

 connected with one of its arms is made to play against a strip of black- 

 ened paper. As the paper remains stationary, the point makes but a 

 single mark ; but when the paper moves, as the point vibrates, a line is 

 produced with regular curves, each curve representing ^-^ of a second. 

 If a lever is now attached to a muscle and so arranged as to indicate 



