392 OF NERVOUS IRRITABILITY 



also, as that given above with regard to the muscles, nervous irri- 

 tability lasts much longer after death in the cold-blooded than in 

 the warm-blooded animals. Various artificial irritants may be em- 

 ployed to call it into activity. Pinching or pricking the exposed 

 nerve with steel instruments, the application of caustic liquids, and 

 the passage of galvanic discharges, all have this effect. The electric 

 current, however, is much the best means to employ for this pur- 

 pose, since it is more delicate in its operation than the others, and 

 will continue to succeed for a longer time. 



The nerve is, indeed, so exceedingly sensitive to the electric cur- 

 rent, that it will respond to it when insensible to all other kinds of 

 stimulus. A frog's leg freshly prepared with the nerve attached, 

 as in Fig. 133, will react so readily whenever a discharge is passed 

 through the nerve, that it forms an extremely delicate instrument 

 for detecting the presence of electric currents of low intensity, and 

 has even been used for this purpose by Matteucci, under the name 

 of the "galvanoscopic frog." It is only necessary to introduce the 

 nerve as part of the electric circuit ; and if even a very feeble cur- 

 rent be present, it is at once betrayed by a muscular contraction. 



The superiority of electricity over other means of exciting nerv- 

 ous action, such as mechanical violence or chemical agents, pro- 

 bably depends upon the fact that the latter necessarily alter and 

 disintegrate more or less the substance of the nerve, so that its irri- 

 tability soon disappears. The electric current, on the other hand, 

 excites the nervous irritability without any marked injury to the 

 substance of the nervous fibre. Its action may, therefore, be con- 

 tinued for a longer period. 



Nervous irritability, like that of the muscles, is exhausted by repeated 

 excitement. If a frog's leg be prepared as above, with the sciatic 

 nerve attached, and allowed to remain at rest in a damp and cool 

 place, where its tissue will not become altered by desiccation, the 

 nerve will remain irritable for many hours ; but if it be excited, 

 soon after its separation from the body, by repeated galvanic shocks, 

 it soon begins to react with diminished energy, and becomes gra- 

 dually less and less irritable, until it at last ceases to exhibit any 

 further excitability. If it be now allowed to remain for a time at 

 rest, its irritability will be partially restored ; and muscular contrac- 

 tion will again ensue on the application of a stimulus to the nerve. 

 Exhausted a second time, and a second time allowed to repose, it 

 will again recover itself; and this may even be repeated several 

 times in succession. At each repetition, however, the recovery of 



