and supporting the vibrations of the particles, not as exciting them originally.
Thirdly, The vibrations thus excited in the æther, and particles of the sensory nerves, will be propagated along the course of these nerves up to the brain. For the æther residing in the medullary substance, being of an uniform density on account of the smallness of the pores of the medullary substance, and uniformity of its texture, before taken notice of, will suffer the excited vibrations to run freely through it. And the same uniformity, together with the continuity, softness, and active powers of the medullary substance, will farther contribute to the free propagation of the vibrations; since, on these accounts, it follows, that the particles, which were last agitated, may easily communicate their agitations or vibrations to the similarly posited and equal contiguous ones, without interruption, and almost without any diminution of force. This free propagation of vibrations along the course of the nerves may be illustrated and confirmed by the like free propagation of sounds along the surface of water, which has sometimes been observed in still, calm nights.
Fourthly, The vibrations here described are confined to the medullary substance, or at least are only propagated feebly and imperfectly into the neighbouring parts, on account of the heterogeneity and geater hardness of the neighbouring parts. The first will make the æther of different densities, and in some cases there may be almost an interruption or discontinuity of it; and the last will indispose the particles to receive and communicate vibrations; and we may suppose from both together, agreeably to what has been already remarked, that only small vibrations, and such irregular ones as oppose each other, will just begin to take place in the immediately contiguous parts, and there cease without proceeding farther. It is somewhat analogous to this in sounds, that they are much sooner lost in passing over rough surfaces than smooth ones; and particularly, that they receive a much greater diminution from the irregular surface of the earth, than from that of still water. However, a particular exception is here to be made in respect of the fibres of the muscles and membranes, into which the vibrations of the æther and medullary particles seem to be propagated with great freedom and strength, as will be seen hereafter. Which may perhaps be some argument that muscular fibres are, according to Boerhaave’s opinion, mere productions of the ultimate nerves.
Fifthly, As soon as the vibrations enter the brain, they begin to be propagated freely every way over the whole medullary substance; being diminished in strength, in proportion to the quantity of matter agitated, just as in sounds, i.e. as it were in a reciprocal duplicate ratio of the distance from the place where the sensory nerve affected by the vibrations enters the brain. Or, if we suppose the pia mater to make some small discontinuity
