brain, and over the whole muscular system, immediately; putting all the muscles into a state of contraction, as much as may be, i.e. making the strongest set of muscles everywhere overpower the weaker, for a certain time, and then give place to them for a certain other time, and so on alternately. Since therefore the muscles of expiration are stronger than those of inspiration, the air will be forced strongly out of the thorax through the larynx, and, by consequence, yield a sound. It contributes to this, that the muscles of the os hyoides and larynx, acting all together, and drawing different ways, must suspend the cartilages of the larynx, so as both to narrow the passage of the air, and also render these cartilages more susceptible of vibrations. As to the muscles which contract and dilate the larynx, they are perhaps about equal in strength to each other, and therefore may, by opposing each other, keep the chink in a state intermediate between its least and greatest dilatation.
That the strongest set of antagonist muscles overpower the weaker, during the great effort in crying, may farther appear from the action of the extensors of the neck, and flexors of the hand (both which sets are stronger than their antagonists), at that time.
If it be objected here, that the elevators of the lower jaw, being stronger than the depressors, ought to keep the mouth shut during the action of crying, according to the foregoing reasoning, whereas the contrary always happens; I answer, first, that when both these sets of muscles act at the same time, in proportion to their natural strength, the mouth ought to be a little opened; secondly, that the vibrations which take place in the cartilages of the larynx seem to impart a peculiar degree of force to all the neighbouring muscles, i.e. to the depressors of the lower jaw; and, thirdly, that the muscles which pass from the larynx and os hyoides to the lower jaw act to an advantage in drawing it down, in the present case, because the os hyoides is at this time fixed by its other muscles.
The distortions of the face, which happen previously to crying, and during the course of it, seem to be sufficiently agreeable to the notions here advanced; the muscles, which draw the lips from each other, being much stronger, than those which close them.
The manner in which titillation occasions laughter in its automatic state, has been already explained. We may add here, that touching the cheeks of young children gently will excite smiling.
Friction also occasions many automatic motions in young children, as may be observed when their naked bodies, or hairy scalps, are rubbed by the nurse’s hand; the motion being determined in these cases, as appears, by the strength and vicinity of the muscles.
The contraction of the hand in young children, which has been taken notice of already, may be excited by titillation, friction, and almost any impression on the palm; and is to be deduced
