NERVOUS IRRITABILITY. 



93 



the nerves is at its maximum, they may be excited by me- 

 chanical, chemical, or galvanic stimulus, all of these agents 

 producing contraction of the muscles to which the motor fila- 

 ments are distributed. Mechanical irritation, simply pinch- 

 ing a portion of the nerve, for example, produces a single 

 muscular contraction; but if the injury to the nerve be such 

 as to disorganize its fibres, that portion of the nerve will 

 no longer conduct a stimulus. Among the irritants of this 

 kind, we may cite the 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 pro- 

 duced by the sudden application of intense cold. Among 

 chemical reagents, there are some that excite the nerves and 

 others which produce no effect ; but these are not important 

 from a physiological point of view. Suffice it to say that 

 mechanical irritation and the action of certain chemicals are 

 capable of exciting the nerves ; but that 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 injury has no effect, stimulation 

 between this point and the muscles is still followed by con- 

 traction. 



The most convenient method of exciting the nerves in 

 physiological experiments is by means of electricity, a stimu- 

 lus more closely resembling the nerve-force than any other, 

 and one which may be employed without disorganizing the 

 nerve-tissue, and consequently admits of extended and re- 

 peated application. The action of electrfcity, however, with 

 the methods of preparing the nerves and muscles for experi- 

 mentation, will be fully considered under a separate head. 



The irritability of the motor system is entirely distinct 

 from that of the sensory nerves, and one may be destroyed, 

 leaving the other intact. This follows almost as a matter of 

 course upon the fact of the anatomical distinction between 

 motor and sensory nerves ; but it is interesting to note the 

 limits of the irritability after death in nerves of different 



