We may draw the following corollaries from the hypothesis of vibrations, as laid down in the two foregoing propositions.
Cor. I. The vibrations of the medullary particles may be affected with four sorts of differences; viz. those of degree, kind, place, and line of direction. Vibrations differ in degree, according as they are more or less vigorous; i.e. as the particles oscillate to and fro, through a longer or shorter very short space; i.e. as the impression of the object is stronger or weaker, and thus affects the medullary particles more or less vigorously, either directly and immediately, or mediately, by generating a greater or less degree of condensation in the pulses of the æther. Vibrations differ in kind, according as they are more or less frequent; i.e. more or less numerous, in the same space of time. They differ in place according as they affect this or that region of the medullary substance of the brain primarily. And they differ in the line of direction, according as they enter by different external nerves.
Cor. II. The magnitude of each sensation is chiefly to be estimated from the vibrations which take place in the medullary substance of the brain; those which are excited in the spinal marrow and nerves being, for the most part, so inconsiderable, in respect of the just-mentioned ones, that they may be neglected.
Cor. III. The brain may therefore, in a common way of speaking, be reckoned the seat of the sensitive soul, or the sensorium, in men, and all those animals where the medullary substance of the nerves and spinal marrow is much less than that of the brain; and this even upon the supposition laid down in the first proposition; viz. that the whole medullary substance of the brain, spinal marrow, and nerves, is the immediate instrument of sensation, and equally related to the sensitive soul, or principle. But if there be any reason to suppose that the first proposition is not strictly true, but that the spinal marrow and nerves are only instruments subservient to the brain, just as the organs of the hand, eye, ear, &c. are to them, and the brain itself to the soul, we may conclude absolutely, that the sensorium of such animals is to be placed in the brain, or even in the innermost regions of it. Now there are some phænomena which favour this, by shewing, that whatever motions be excited in the nerves, no sensation can arise unless this motion penetrate to, and prevail in, the brain. Thus, when a nerve is compressed, we lose the sense of feeling in the parts to which it leads: a person much intent upon his own thoughts does not hear the sound of a clock; i.e. the vibrations excited by this sound in the auditory nerve cannot penetrate to, and prevail in, the brain, on account of those which already occupy it: and a person who has lost a limb often feels a pain which seems to proceed from the amputated limb; probably because the region of the brain corresponding to that limb is still affected.
If it be certain, that some of the medullary parts have been
