AXD ITS MODE OF ACTION. 393 



nervous irritability is less complete, until it finally disappears alto- 

 gether, and can no longer be recalled. 



Various accidental circumstances tend to diminish or destroy 

 nervous irritability. The action of the woorara poison, for example, 

 destroys at once the irritability of the nerves ; so that in animals 

 killed by this substance, no muscular contraction takes place on 

 irritating the nervous trunk. Severe and sudden mechanical inju- 

 ries often have the same effect ; as where death is produced by 

 violent and extensive crushing or laceration of the body or limbs. 

 Such an injury produces a general disturbance, or shock as it is 

 called, which affects the entire nervous system, and destroys or 

 suspends its irritability. The effects of such a nervous shock may 

 frequently be seen in the human subject after railroad accidents, 

 where the patient, though very extensively injured, may remain 

 for some hours without feeling the pain of his wounds. It is only 

 after reaction has taken place, and the activity of the nerves has 

 been restored, that the patient begins to be sensible of pain. 



It will often be found, on preparing the frog's leg for experiment 

 as above, that immediately after the limb has been separated from 

 the body and the integument removed, the nerve is destitute of 

 irritability. Its vitality has been suspended by the violence in- 

 flicted in the preparatory operation. In a few moments, however, 

 if kept under favorable conditions, it recovers from the shock, and 

 regains its natural irritability. 



The action of the galvanic current upon the nerves, as first shown 

 by the experiments of Matteucci, is in many respects peculiar. If 

 the current be made to traverse the nerve in the natural direction 

 of its fibres, viz., from its origin towards its distribution, as from a 

 to I in Fig. 133, it is called the direct current. If it be made to 

 pass in the contrary direction, as from b to a, it is called the inverse 

 current. When the nerve is fresh and exceedingly irritable, a 

 muscular contraction takes place at both the commencement and 

 termination of the current, whether it be direct or inverse. But 

 very soon afterward, when the activity of the nerve has become 

 somewhat diminished, it will be found that contraction takes place 

 only at the commencement of the direct and at the termination of the 

 inverse current. This may readily be shown by preparing the two 

 legs of the same frog in such a manner that they remain connected 

 with each other by the sciatic nerves and that portion of the spinal 

 column from which these nerves take their origin. The two legs, 

 so prepared, should be placed each in a vessel of water, with the 



