solutions of continuity, occasioned by wounds, burns, &c. there always arises in the neighbouring parts, which are inflamed, a preternatural distention of the small fibres and vessels; by which means the pain is renewed and continued. Every manifest solution of continuity does therefore, according to the explanation of distention just laid down, include within itself an infinite number of minute invisible solutions.
Hence we may ask, whether this minute invisible solution of continuity in the infinitesimal medullary particles of the brain be not that common limit, and middle point, which separates pleasure from pain, and of which the visible solutions of continuity, which are caused by external injuries, are a type, and also a means, viz. by propagating violent vibrations up to the brain? It is some presumption in favour of this position, that all conjectures concerning invisible things ought to be taken from visible ones of the same kind; also, that it is particularly suitable to the doctrine of vibrations; inasmuch as, laying down this doctrine, one may easily conceive how moderate and pleasant impressions may agitate the medullary particles in so moderate a degree, as that they shall again return to their former situations and connexions, when the agitation is over; whereas violent and painful ones may force the particles from thence, and give rise to new ones, i.e. to the solution of continuity. And as the body is so formed, that great and visible solutions of continuity may be healed again, and the parts restored, in great measure, to their primitive integrity and perfection, by the power of nature, unless where there is a loss of substance, (and yet even here the same end is obtained in part;) so we may suppose, that the power of nature restores all minute solutions of continuity in the constituent infinitesimal particles almost instantaneously, and so that the body receives no perceptible detriment from single instances, though it probably does from frequent repetitions; agreeably to which, it is generally supposed, that pain, by often returning, impairs the faculties, both bodily and mental.
Fourthly, The bones, nails, hair, and cuticle, may, consistently with the doctrine of vibrations, have a solution of continuity produced in their parts, without pain ensuing; for they are hard, and therefore incapable, as it seems, of receiving and communicating to the contiguous nerves, and thence to the brain, vibrations of any considerable degree of strength. We are also to suppose, that in palsies, mortifications, &c. changes of texture of somewhat a like kind are produced, so as to render the parts affected thereby incapable of conveying sensation to the medullary substance of the brain. Old age, inactivity, inflammation, pain, &c. are in like manner to be considered, as inducing such a degree of condensation, fixation, and callosity, in the medullary substance itself, as must end at last in insensibility and death.
Fifthly, It is not unsuitable to the doctrine of vibrations, that the frequent repetition of the same external impressions should
