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occasion merely direct your attention to the consideration of one other subject, which is, the opinions we may be warranted in forming, respecting the connection of irritability and sensibility.
f , This subject has been the cause of much controversy. Haller maintained that irritability was a distinct property inherent in muscles ; to use his own words, that they had a vis insita, independent of the vis nervea; which opinion has of late received additional corroboration from some experiments of Mr. Brodie. Those who object to this opinion, can, I think, only oppose it on the following grounds. They must contend either that the muscles have a kind of perception of injury which causes them to contract, even though they are unconnected with the brain; or that the nerves are the organs, which prepare and supply the muscles with
something which is the cause of irritability.
Concerning the first of these suppositions, that muscles may have a perceptibility of injury, distinct from that which we understand to be feeling, I have to observe, that we can have no idea of sensation but what results from our own experience, which may be defined to be perception attended with consciousness; which kind of sensation is confined to the brain alone. Of any other kind of perception, it is evident we can never form any idea.
If a man's leg be amputated, and by
voltaic electricity I excite contraction in
its muscles for some hours, how can I
know whether they feel or not? We na
turally judge of other subjects from our
selves, and knowing that we shrink from G
whatever pains us, some persons seem to conclude that the muscles contract because they have been hurt. To the patient who has suffered amputation, such a supposition would seem absurd. He may feel pain when no stimulus is applied to the limb, or he may feel case when it is. Nay, he continues to feel pain, or sensations, in the limb when it is rotten, or no longer in existence ; which seems
to shew the integrity of the sentient principle remaining in the brain.
In vegetables, and in some moluscae, no traces of a nervous system are discoverable, yet the irritability of life is manifest in all. In the ascending series of animals, in proportion as the brain becomes large and complex, we have evidence of the perceptions and intelligence increasing; a circumstance which would lead us to believe that these faculties were connected with that part of the nervous system. We have also equal reason to believe, that neither such perception nor intelligence is requisite for the mere functions of life, for these appear to be carried on as effectually in animals that have no brains, nay, in those which seem destitute of any nervous system, as in those which possess such organs. Indeed, many of the most vivacious and irritable animals have the least nervous system. The nerves in the lower order of animals, that have no common sensorium, may contribute to produce effects, which, in tracing the ascending series, I have endeavoured to express by the words concurrence of impressions and actions ; because intimations of impressions and actions occurring in one part may be communicated to others by these inter-nunciate chords, as Mr. Hunter called them, in cases where we G 2
are not warranted in supposing there is any sensation such as I have defined.
Assuredly, motion does not necessarily imply sensation; it takes place where no one ever yet imagined there could be sensation. If I put on the table a bason containing a saturated solution of salt, and threw into it a single crystal; the act of crystallization would begin from the point touched, and rapidly and regularly pervade the liquor till it assumed a solid form. Yet I know I should incur your ridicule, if I suggested the idea that the stimulus of the salt had primarily excited the action, or that its extension was the effect of continuous sympathy. If also I threw a spark amongst gun-powder, what would you think were I to represent the explosion as a struggle resentful of injury, or the noise as the clamorous expression of pain?