YAWNING is an involuntary opening of the mouth; which is occasioned by weariness, or an inclination to sleep; though it frequently occurs early in the morning, and immediately after rising.
Yawning is performed by extending all the muscles, that are capable of spontaneous motion; by expanding the lungs; by slowly and gradually inhaling a large portion of air, and expiring or breathing it out, after it has been rarefied in the body; and lastly, by restoring the muscles to their natural position. Persons of strong and healthy constitutions are most liable to this natural impulse, when they awake, on account of the perspirable matter that lodges in the pores of the skin: hence Boerhaave observes, that yawning appears to be designed by Nature, to move, accelerate, and uniformly to distribute the humours throughout the system; consequently, to render the various muscles capable of discharging their respective functions. Such involuntary motion, therefore, greatly conduces to health; but, if it become habitual, or be too often repeated, especially by persons of sedentary lives, it will be advisable to take a short walk in the fresh air, and to inspire a large portion of that salutary fluid, in temperate weather. Sometimes, however, yawning is the fore-runner of convulsions or ague-fits; in which cases, those distressing affections might often be prevented, by adopting timely and proper remedies.