gentle contraction in the sphincter ani, viz. such a one as does not exhaust its power, and which therefore tends to confine the fæces. In like manner, an irritation in the rectum may gently contract the upper orifice of the stomach. It deserves notice here, that the sphincter ani lies out of the peritonæum; and consequently, that vibrations cannot run from it to the orifice of the stomach along the peritonæum, nor vice versâ. The same observation holds in respect of the sphincter of the bladder.
The circumstances attending the exclusion of the fœtus, which continues automatic perfectly or nearly, may be much illustrated by what is here delivered concerning the expulsion of the fæces.
When young children continue to suck, or masticate a tasteless body put between their lips, or into their mouths, we may conceive, that the actions of suction and mastication begin to pass from their automatic towards their voluntary state. Drinking out of a vessel, so as to draw up the liquid, is learnt, in part, from sucking the breast, in most cases; but it may be learnt without, as is evident in those children that are dry-nursed. Mastication, when it approaches to a perfect state in children, is chiefly voluntary, the first rudiments receiving perpetual changes, so as to fall in more and more with pleasure and convenience. See Prop. XXII. Cor. I. In adults, both suction and mastication follow the command of the will with entire readiness and facility. The manner in which this is effected has been already explained in treating of the voluntary power of grasping.
Deglutition of insipid liquors becomes voluntary early. But it is difficult even for some adults, to swallow pills and boluses, though tasteless; and very nauseous liquids are sometimes rejected by them automatically, as well as by young children. The action of deglutition affords manifest evidences of the gradual transition of automatic motions into voluntary ones, as well as of voluntary ones into such as are secondarily automatic. For, in common cases, we swallow without the least express intention.
When the face of a child or adult is distorted upon the sight of a nauseous medicine, which has before produced distortions automatically, i.e. from the impressions made on the mouth and fauces, we see an evident instance of the power of associated circumstances; and may have the conception of voluntary powers, derived from a succession of such associations, made easy to the imagination.
The peristaltic motion of the stomach and bowels remains automatic to the last, depending partly on the vibrations descending from the brain, partly on the impressions made on the
