NERVOUS EXCITABILITY. 527 



consideration ; and anaesthetics temporarily abolish the physiological proper- 

 ties of the sensory nerves without necessarily affecting the motor nerves. 



Until the results of experiments upon the artificial union of motor 

 and sensory nerves become much more positive than they now are, it must 

 be assumed that these two kinds of nerve-fibres have distinct physiological 

 properties, both as regards the kind of impulse or impression produced by 

 excitation or stimulation and the direction of conduction. It is possible, 

 however, that these properties may be modified by altered relations for a 

 long time with the trophic centres that influence the nutrition of the differ- 

 ent kinds of nerve-fibres. 



Nervous Excitability. Immediately or soon after death, when the excit- 

 ability of the nerves is at its maximum, they may be stimulated by mechani- 

 cal, chemical or galvanic irritation, all of these agents producing contraction 

 of the muscles to which the motor filaments are distributed. Mechanical irri- 

 tation, simply pinching a portion of the nerve, for example, produces a single 

 muscular contraction ; but if the injury to the nerve be such as to disorgan- 

 ize its fibres, that portion of the nerve will no longer conduct an impulse. 

 Among the irritants of this kind, are extremes of heat and cold. If an ex- 

 posed nerve be cauterized, 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 stimulation 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, irritation 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 most convenient method of exciting the nerves in physiological ex- 

 periments is by means of electricity. This may be employed without dis- 

 organizing 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 ex- 

 tended and improved by Marey. The process employed by Marey, which is 

 essentially the same as that used in all 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 blackened 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, 



