disorders of the brain, and its influence over the fifth pair of nerves, do not take place till intellectual aggregates are formed. And the like reason may hold in respect of brutes.
The actions of sobbing and weeping are therefore, in part, deducible from association; i.e. are not merely automatic, in the first sense of that word. Agreeably to which, they are in certain cases manifestly subject to the voluntary power. Thus, some persons can, by introducing imaginary scenes of compassion and sorrow, so far agitate the brain, as to bring on the actions of sobbing and weeping, though not in the same degree, as when they arise from a strong real mental cause. They may likewise be caught by infection, from others, as laughter, and most of our other semi-voluntary and voluntary actions, are; which is another argument of their dependence on association.
Since strong vibrations must, according to the foregoing theory, descend at once into the whole muscular system in general convulsions, we must seek for a cause of sufficient extent for this purpose. Now there seem to be three kinds of vibrations, which may answer this condition: first, violent vibrations in the brain. Secondly, violent ones at the skin, suddenly checked. Thirdly, violent ones in the bowels or uterus, suddenly checked also, and thence running instantaneously over the whole nervous system by means of the intercostal, or, as Winslow justly calls it, the great sympathetic nerve.
Convulsions from compressions and inflammations of the brain, and most of those which are termed epileptic, seem to be of the first kind. In epilepsies the irregular vibrations, excited in the medullary substance of the brain, are perhaps so violent, as first to make the small particles attract each other, and thus, by checking themselves, to extinguish all sense and motion. However, they may return after a short time, and descend into the whole muscular system.
The stretchings and yawnings which happen in ague-fits, in going to sleep and waking, the startings to which some persons are subject in going to sleep, and the convulsive tremors and rigidity in ague-fits, seem to be of a second kind, or to arise from a sudden check of vibrations at the skin. For in agues the surface is chilled, as it is also by the least motions in going to sleep, or waking. Agreeably to this, it may be observed, that, upon stepping into a cold bed, one is disposed to general convulsions, like those of stretching. Yawning may also depend in part upon a like check of violent vibrations in the mouth and fauces; for it is a motion excited in the neighbourhood, and is observed to accompany sickness.
