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. Humsotpt 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 
neryous energy becomes exhausted, and the death of the 
individual ensues. If fatigued only in a moderate degree, the 
