chiefly on the sensations of the lips and tongue. I say chiefly, because some predisposition thereto may be generated in utero, or otherwise impressed, and the great aptness of new-born children in sucking seems to favour this. However, when we consider, that the impressions of the cold air upon the lips and mouth in its passage to the lungs, of the nipple upon the lips, and of the milk upon the tongue, ought to excite motions in the neighbouring muscles of the lips, and lower jaw; that the motions which concur to the action of suction, are such as might be expected from these causes; and even that the motions of the head and neck, by which the child indicates the want of a breast, may flow from the great sensibility and irritability of these parts, when the child is hungry; a presumption arises, that the whole action of suction, with all its circumstances, is excited by the impressions mechanically or automatically; and that by the running of vibrations from the sensory nerves into the neighbouring motory ones.
Secondly, The first rudiments of the action of mastication are derived from that of suction, i.e. from the alternate motion of the lower jaw necessary to squeeze out the milk. After this action has been excited for some time by the taste of the milk, it will return with sufficient facility from the impressions made by solid food; and the same impressions may excite other motions in the muscles of the tongue and cheeks, viz. those which concur to make the action of mastication in its imperfect and automatic state.
Thirdly, It may appear in like manner, that the pleasurable impressions of the milk upon the tongue, mouth, and fauces, of new-born children, may excite those motions of the muscles of the tongue, os hyoides, soft palate, and fauces, which make the action of deglutition; and consequently, that this is deducible from sensation automatically.
It confirms this position, that, according to the theory of these papers, the soft palate ought to be drawn down by the impressions made on the tongue and mouth, not drawn up; since this last would be to suppose the sensory vibrations to pass over muscles that are near, and run to those at a distance, which is absurd. For Albinus has proved, both from anatomy, and the observation of the fact, that the soft palate is drawn down in swallowing; not up, according to the opinion of Boerhaave.
It confirms it also, that nauseous liquids are immediately and mechanically rejected by young children; the impressions arising from them producing such a contraction as shuts the passage.
It confirms it still farther, that young children do not swallow their saliva. For this makes no impression sufficient to generate the action of deglutition in an automatic way.
We may conjecture here, that the common vibrations, excited in the membrane of the mouth and fauces, grow particularly strong at the tip of the uvula; and that a greater power of
