substance be mixed with the medullary in the brain, and spinal marrow, and perhaps in the ganglions and plexuses, yet it does not appear that the communication of any one part of the medullary substance with every other, is cut off any where by the intervention of the cortical. There is no part of the medullary substance separated from the rest, but all make one continuous white body; so that if we suppose vibrations apt to run freely along this body from its uniformity, they must pervade the whole, in whatever part they are first excited, from its continuity.
The excessive minuteness of the vessels of which the medullary substance consists, may also be conceived as inferring its uniformity and continuity. These vessels are, by all anatomists and physiologists, supposed to arise from those of the cortical substance, this being agreeable to the analogy of the other parts of the body. And it follows from the same analogy, that they must be smaller than those vessels from which they arise. But the finer orders of the vessels of the cortical substance are far too minute to admit of the most subtle injections, the best injectors having never penetrated farther than the grosser orders of vessels in the cortical substance. We may therefore well suppose, that the medullary substance consists of a texture of vessels so small and regular, as that it may have no vacuity or interval in it, sufficient to interrupt or disturb the vibrations of the æther, and concomitant ones of the small medullary particles, propagated through this substance in the manner to be described below.
The softness of the medullary substance is, in like manner, evident to the senses, and the natural consequence of the extreme smallness of the compounding vessels, and fluids circulating through them.
If we admit the foregoing account of the uniform continuous texture of the medullary substance, it will follow that the nerves are rather solid capillaments, according to Sir Isaac Newton, than small tubuli, according to Boerhaave. And the same conclusion arises from admitting the doctrine of vibrations. The vibrations hereafter to be described, may more easily be conceived to be propagated along solid capillaments, so uniform in their texture as to be pellucid when singly taken, than along hollow tubuli. For the same reasons, the doctrine of vibrations will scarce permit us to suppose the brain to be a gland properly so called; since the deformity of texture required in a gland appears inconsistent with the free propagation of vibrations. Neither can we conclude the brain to be a gland, from the great quantity of blood sent to it by the heart. It is probable, indeed, that this is required on account of the important functions of accretion, nutrition, sensation, and motion, which are plainly performed by the brain. But then these functions admit of as easy an explanation on the hypothesis here proposed, as on that of a glandular secretion, called nervous fluid, animal spirits, &c.
