in the medullary substance by its processes, as has been hinted above, then we must also suppose that the vibrations, which ascend along any sensory nerve, affect the region of the brain which corresponds to this sensory nerve more, and the other regions less, than according to this proportion.
Sixthly, Since the vibrations, or reciprocal motions, of the small particles of each nerve are made in the same line of direction with the nerve, they must enter the brain in that direction, and may preserve some small regard to this direction at considerable distances within the brain; especially if this be favoured by the structure of the nervous fibrils in the brain. Hence the same internal parts of the brain may be made to vibrate in different directions, according to the different directions of the nerves by which the vibrations enter.
And thus it appears, that, admitting the existence and subtlety of the æther, and the qualities of the medullary substance here alleged, a probable account may be given, how the vibrations, asserted in the last proposition, may be excited in the sensory nerves, and propagated thence over the whole medullary substance, and over that alone. And the suitableness of this proposition to the last, and of both to a variety of phænomena, which will be seen in the course of these observations, may be considered as some evidence for both.
Let it be remarked also, that, if the performance of sensation by vibratory motions of the medullary particles be admitted, the existence of a subtle elastic fluid must be admitted in consequence thereof, as the only means that can be conceived for their rise and free propagation, so as to answer to the phænomena of sense, motion, and ideas; and reciprocally, if the existence of so subtle and elastic a fluid, as the æther described by Sir Isaac Newton, can be established upon independent principles, it may reasonably be supposed to penetrate the pores of the medullary substance, how small soever they be, in the same manner as air penetrates grosser cavities and pores; and, like air, both be itself agitated by vibrations from a variety of causes, and also communicate these to the medullary particles. We may, therefore, either deduce the doctrine of vibrations here proposed from the consideration of the æther, or the existence of the æther from the doctrine of vibrations, according as either of these can be first established.
There is also some light and evidence to be cast upon one or both of these propositions, from several natural phænomena; as I will endeavour to shew in the following remarks:—
1. Heat in natural bodies is probably attended by vibrations of the small parts. This may be inferred from the duration and gradual declension of heat, and from the gross general proportion which is observed between this duration and the density of the heated body. For a vibratory motion would subsist for some time, decline gradually, and be kept up longer, cæteris paribus,
