AND ITS MODE OF ACTION. 421 



and destroys or suspends its irritability. The effects of a nervous 

 shock of this kind may frequently be seen in man after railroad acci- 

 dents, where the patient, though extensively injured, may remain for 

 some hours in a state of unusual muscular debility, and at the same time 

 without the sensation of pain. It is only after reaction has taken place, 

 and nervous irritability has been restored by repose, that the powers of 

 sensation and voluntary motion are re-established. 



It is often 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 inflicted in the preparatory 

 operation. In a few moments, if kept under favorable conditions, it 

 recovers from the shock, and regains its natural irritability. 



Different Action of the Direct and Inverse Currents. The action 

 of the galvanic current upon the nerves, as first shown by Matteucci, is 

 in many respects peculiar. If the current be made to traverse the nerve 

 in the natural direction of its fibres, namely, from its origin toward its 

 distribution, as from a to b (Fig. 144), 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, 

 or when the galvanic current is of sufficient intensity, a muscular con- 

 traction takes place at both the commencement and termination of the 

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

 nerve has become somewhat diminished, or when the current emploj^ed 

 is of feeble intensity, 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 



Pi?. 145. 



and that portion of the spinal column from which these nerves take their 

 origin. The two legs, so prepared, are placed each in a vessel of water, 

 with the nervous connection hanging between. (Fig. 145.) If the posi- 

 tive pole, a, of the battery be now placed in the vessel which holds leg 



