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These four faculties of the sensorium during their inactive state are termed irritability, sensibility, voluntarity, and associability; in their active state they are termed as above, irritation, sensation, volition, association. IRRITATION is an exertion or change of some extreme part of the sensorium residing in the muscles or organs of sense, in consequence of the appulses of external bodies.
For the vivacity of these trains of ideas, passing in the imagination, is greatly increased by the causes above-mentioned; that is, by their not being disturbed or dissevered either by the appulses of external bodies, as in surprize; or by our voluntary exertions in comparing them with our previous knowledge, of things, as in reasoning upon them. B. Now to apply.
Hence I conclude, that our nerves of sense are not torpid or inert during sleep; but that they are only precluded from the perception of external objects, by their external organs being rendered unfit to transmit to them the appulses of external bodies, during the suspension of the power of volition; thus the eye-lids are closed in sleep, and I suppose the tympanum of the car is not stretched, because they are deprived of the voluntary exertions of the muscles appropriated to these purposes; and it is probable something similar happens to the external apparatus of our other organs of sense, which may render them unfit for their office of perception during sleep: for milk put into the mouths of sleeping babes occasions them to swallow and suck; and, if the eye-lid is a little opened in the day-light by the exertions of disturbed sleep, the person dreams of being much dazzled.
Irritative motions. That exertion or change of the sensorium, which is caused by the appulses of external bodies, either simply subsides, or is succeeded by sensation, or it produces fibrous motions; it is termed irritation, and irritative motions are those contractions of the muscular fibres, or of the organs of sense, that are immediately consequent to this exertion or change of the sensorium.
If nevertheless this theory should ever become established, a stimulus must be called an eductor of vital ether; which stimulus may consist of sensation or volition, as in the electric eel, as well as in the appulses of external bodies; and by drawing off the charges of vital fluid may occasion the contraction or motions of the muscular fibres, and organs of sense.
Every man should be so much an artist that he could report in conversation what had befallen him. Yet, in our experience, the rays or appulses have sufficient force to arrive at the senses, but not enough to reach the quick and compel the reproduction of themselves in speech.
When we are employed with great sensation of pleasure, or with great efforts of volition, in the pursuit of some interesting train of ideas, we cease to be conscious of our existence, are inattentive to time and place, and do not distinguish this train of sensitive and voluntary ideas from the irritative ones excited by the presence of external objects, though our organs of sense are furnished with their accustomed stimuli, till at length this interesting train of ideas becomes exhausted, or the appulses of external objects are applied with unusual violence, and we return with surprise, or with regret, into the common track of life.
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