I mean likewise, that when any organ of sense is for some time affected in some one manner, if it be suddenly affected otherwise, there ensues a convulsive motion; such a convulsion as is caused when anything happens against the expectance of the mind.
And though it may appear strange that such a change as produces a relaxation should immediately produce a sudden convulsion; it is yet most certainly so, and so in all the senses.

Every one knows that sleep is a relaxation; and that silence, where nothing keeps the organs of hearing in action, is in general fittest to bring on this relaxation; yet when a sort of murmuring sounds dispose a man to sleep, let these sounds cease suddenly, and the person immediately awakes; that is, the parts are braced up suddenly, and he awakes.

This I have often experienced myself, and I have heard the same from observing persons.

In like manner, if a person in broad daylight were falling asleep, to introduce a sudden darkness would prevent his sleep for that time, though silence and darkness in themselves, and not suddenly introduced, are very favorable to it.

This I knew only by conjecture on the analogy of the senses when I first digested these observations; but I have since experienced it.