contraction is transmitted to the neighbouring muscles upon this account.
Fourthly, It may be observed, that nauseous tastes distort the mouth and face automatically, not only in young children, but even in adults. And, for the same reason, pleasant ones ought to have a less effect, of the same kind; as they seem to have. And I conjecture, that the distortions of the face, which attend grief, also the gentle, smiling motions, which attend joy, are, in part, deducible from this source.
I conjecture also, that the risus sardonius, and the tendency to laughter, which some persons observe in themselves in going to sleep, have a relation to the fore-mentioned motions of the face. As the muscles here considered are, in great measure, cutaneous, they will, on this account, be more subject to vibrations excited in the mouth, or which run up to it from the stomach.
Fifthly, It may easily be conceived, that the impressions, which the aliment and fæces make upon the stomach and bowels, may excite the peristaltic motion in their muscular coats. It only remains to shew, why this should tend downwards. Now, for this, we may assign the following reasons. First, that the action of swallowing determines that of the stomach to move the same way with itself, i.e. downwards; and that this determination may, in common cases, carry its influence as far as the great guts. Secondly, that the contraction of the upper orifice of the stomach may stop the waves that sometimes come upwards in the stomach, and return them back, so as to force open the pylorus where that is less contracted; as on the other hand, where the pylorus is more contracted than the upper orifice, the motion of the stomach is inverted, and there arises a disposition to ructus or vomiting. Thirdly, that when waves ascend in the lower bowels, a gentle contraction in the pylorus may be sufficient to stop and return them. Fourthly, that one principal use of the cæcum, and appendicula vermiformis, which last is an extreme and pointed part, seems to be, to return the waves, which the constriction of the anus may send upwards. And the effects of glysters and suppositories in procuring stools, i.e. in putting the whole colon into motion, agree well with this use of the cæcum, and appendicula vermiformis. It agrees also with all the reasoning of this paragraph, that when a stoppage is made any where in the bowels by an inflammation, spasm, strangulation from a rupture, &c. the peristaltic motion is inverted.
I have been informed, that in a person who had some inches of the ilium hanging out of his body, so that the peristaltic motion might be viewed, the least touch of a foreign body would stop this motion at once. It agrees with this, that when rabbits are opened alive, the peristaltic motion does not take place till after some time, viz. because the handling of the bowels has checked it. May we not hence suspect, that the fibres of the muscular coat
