"Master, I have brought unto thee my son, which hath a dumb spirit; and wheresoever he taketh him, he teareth him: and he foameth, and gnasheth with his teeth, and pineth away" (Mark 9:17-18).
Why was he foaming at the mouth, gnashing his teeth and withering away? Of all the possessed man's body, his brain suffers first and foremost, since the demon uses the spirit of the soul within the brain as a vehicle, and from there, as from a citadel, exercises power over the whole body. When the brain is afflicted, it emits a frothy, phlegm-like discharge to the nerves and muscles of the body, blocking up the outlets of the soul's spirit. As a result, shock, convulsions and involuntary movements affect all the parts of the body capable of independent movement, especially the jaws, as they are nearest to the part originally afflicted. A lot of moisture is brought down into the mouth due to the size of its pores and its proximity to the brain. Since, because of the unruly movements of the body's organs, it is impossible to breathe out in one long breath, and the breath is mixed up with the accumulation of moisture, those afflicted foam at the mouth. So the demon was foaming at the mouth and gnashing his teeth, clashing them together horribly and grinding them in a frenzy. The boy was withering away because of the demon's extreme violence. The heat of the sun's rays cause causes mist to form, but if this heat intensifies, it makes the mist disappear and disperses it completely. Similarly, the demon's violence causes moisture to come from the internal organs, but if this violence intensifies, the body's natural juices soon evaporate and the possessed person withers away.