Page:Observations on Man 1834.djvu/42

Prop. IV.—External Objects impressed upon the Senses occasion, first in the Nerves on which they are impressed, and then in the Brain, Vibrations of the small, and as one may say, infinitesimal, medullary Particles.

These vibrations are motions backwards and forwards of the small particles; of the same kind with the oscillations of pendulums, and the tremblings of the particles of sounding bodies. They must be conceived to be exceedingly short and small, so as not to have the least efficacy to disturb or move the whole bodies of the nerves or brain. For that the nerves themselves should vibrate like musical strings, is highly absurd; nor was it ever asserted by Sir Isaac Newton, or any of those who have embraced his notion of the performance of sensation and motion, by means of vibrations.

In like manner we are to suppose the particles which vibrate, to be of the inferior orders, and not those biggest particles, on which the operations in chemistry, and the colours of natural bodies, depend, according to the opinion of Sir Isaac Newton. Hence, in the proposition, I term the medullary particles, which vibrate, infinitesimal.

Now that external objects impress vibratory motions upon the medullary substance of the nerves and brain (which is the immediate instrument of sensation, according to the first proposition) appears from the continuance of the sensations mentioned in the third; since no motion, besides a vibratory one, can reside in any part for the least moment of time. External objects, being corporeal, can act upon the nerves and brain, which are also corporeal, by nothing but impressing motion on them. A vibrating motion may continue for a short time in the small medullary particles of the nerves and brain, without disturbing them, and after a short time would cease; and so would correspond to the above-mentioned short continuance of the sensations; and there seems to be no other species of motion that can correspond thereto.

Cor. As this proposition is deduced from the foregoing, so if it could be established upon independent principles, (of which I shall treat under the next,) the foregoing might be deduced from it. And on this supposition there would be an argument for the continuance of the sensations, after the removal of their objects; which would extend to the senses of feeling, taste, and smell, in the same manner as to those of sight and hearing.

Prop. V.—The Vibrations mentioned in the last Proposition are excited, propagated, and kept up, partly by the Æther, i.e. by a very subtle and elastic fluid, and partly by the Uniformity, Continuity, Softness, and active powers of the medullary Substance of the Brain, Spinal Marrow, and Nerves.

This proposition is chiefly an evidence and explanation of the foregoing; and accordingly might have been included in it.