158 PHILOSOPHY OF ZOOLOGY. 



the volition be generated in the brain itself, or produced 

 by the sensitive faculty, the action excited in the nerve ap- 

 pears to be the same. How this effect is produced, we are 

 ignorant. There is no reason to consider it as the effect of 

 the concussion of the brain on the nerves, producing vibra- 

 tion, but there is some reason for regarding it as the result 

 of a change in the substance of the nerve, occasioned by the 

 action of the brain. 



Various substances applied to the brain, weaken its power 

 of exciting this action in the nerves. But the same action 

 may be excited in the nerves, independent of the brain. 

 Thus, after the nerve has been separated from the brain, 

 the galvanic fluid will so far excite its energies, as to throw 

 the muscles, to which it is distributed, into as violent mo- 

 tions as if these had been produced by the ordinary process 

 of volition. M. HUMBOLDT has employed this method in 

 distinguishing the nerves from the small vessels. He uses 

 two needles, one gold, another silver. A point of one is 

 applied to the muscles, and a point of the other to the fila- 

 ment, the nature of which he wishes to discover, while the 

 other extremities of these instruments are brought in con- 

 tact. If the filament be a nerve, contractions immediately 

 take place in the muscular fibre. 



In whatever manner the impression is produced, it is pro- 

 pagated along the nerve in a direction opposite to that of 

 sensation, being from the sensorium to the external organs. 

 Its progress, however, is liable to be interrupted by the 

 same causes as those which obstruct the propagation of sen- 

 sation, namely ligature or section. 



The powers of volition appear, like those of sensation, to 

 be liable to fatigue. The continued effort of the will be- 

 comes painful, and if prolonged beyond a limited time, the 

 nervous energy becomes exhausted, and the death of the 

 individual ensues. If fatigued only in a moderate degree, the 



