We come, in the last place, to consider what active properties may belong to the small particles of the medullary substance, i.e. to the small particles which compose either the ultimate vessels of this substance, or the fluid which circulates in these ultimate vessels. The common doctrine concerning the powers of the nervous system supposes the fluid secreted by, and circulating through, the medullary substance to be of a very active nature; and this may be, though the taste of the medullary substance in brute animals discovers no such activity. For the power of impressing tastes seems to reside in particles much larger than those which we are here considering. And it is sufficiently obvious, that many poisons, mineral, vegetable, and animal, have the most active properties concealed under insipid or at least moderate savours. Now that some powers of attraction or repulsion, or rather of both at different distances, reside in the small particles of the medullary substance, can scarce be doubted, after so many instances and evidences, as Sir Isaac Newton has produced, of attractive and repulsive powers in the small particles of various bodies, Opt. Qu. 31; meaning, as he does, by attraction and repulsion, a mere mathematical tendency to approach and recede, be the cause what it will, impulse, pressure, an unknown one, or no physical cause at all, but the immediate agency of the Deity. The smallness also of the particles of the medullary substance may not improbably increase their activity, in respect of their bulk, agreeably to Sir Isaac Newton’s conjecture concerning the particles of the æther. Which may be farther inferred from the nature of these attractions and repulsions; for since they seem to be as some reciprocal power of the distance, we may judge that only the nearest parts of large particles will be eminently active, and that the more remote ones will be an impediment to their actions; whence small particles, having nearly as great active powers, and much less matter to be moved, will, upon the whole, be more active in proportion to their bulk, than large ones. If we farther suppose the particles of the fluids, which circulate through the ultimate vessels of the medullary substance, to be smaller than the particles which compose these vessels, then will they also be more active. And thus we seem to approach to all that is probable in the received doctrines concerning the nervous fluid, and the animal spirits, supposed to be either the same or different things; and all the arguments which Boerhaave has brought for his hypothesis, of a glandular secretion of a very subtle active fluid in the brain, may be accommodated to the Newtonian hypothesis of vibrations.
Having thus endeavoured to settle our notions concerning the æther, and establish our evidences for its existence and properties, and for the uniformity, continuity, softness, and active powers of the medullary substance; we come, in the next place, to inquire in what manner these may serve to explain or evince
