Here we must lay down previously, that the uterus is in a state of distention during pregnancy; and that it propagates sympathetic influences by means of nervous communications to the stomach, so as to put it into a state of great sensibility and irritability. All this will be easily acknowledged.
It follows therefore, since the limits of pleasure and pain are contiguous, that the stomach during pregnancy may at some times have an eager appetite for food, as well as a nausea at others; that this appetite may be the more eager, because it borders upon a nausea; and that it will no more answer to the usual exigencies and circumstances of the body, than the nausea does. The same eager appetite will bring up the ideas of various aliments from prior associations; and if a new association of it, when particularly eager, happen to be made with this or that food or liquor, the sympathetic eager appetite will ever after bring in the idea of that food or liquor, and adhere inseparably to it. The same eager appetite may also be transferred upon something that is not properly a food, from its exorbitant nature, prior nauseas in respect of common food, and accidental joint appearance. And, upon the whole, the usual circumstances attending the longings of pregnant women are deducible from association, and are as agreeable to the doctrine of vibrations, as to any other yet proposed; or even more so.
It may illustrate this account to observe, that in the usual cases of melancholy madness, an uneasy state seems to be introduced into the white medullary substance of the brain by the degeneration of the humours, or other such-like mechanical cause, which carries the vibrations within the limits of pain, and raises an inflammation sui generis in the infinitesimal vessels of the medullary substance; that ideas of objects of fear, sorrow, &c. are raised, in consequence of this, by means of prior associations; and that after some time, some one of these, by happening to be presented oftener than the rest, by falling more in with the bodily indisposition, &c. overpowers all the rest, excites and is excited by the bodily state of fear, sorrow, &c. till at last the person becomes quite irrational in respect of this one idea, and its immediate and close associates, however rational he may be in other respects. And a like account may be given of the violent particular desire towards a person of a different sex, where this desire rests chiefly in the sensual gratification, and the beauty of the person. And all these three instances seem to me to favour the doctrine of vibrations a little, as well as that of association very much.
