16. And this seems to me to be the cause of this spontaneous respiration of ours; for often the mind is occupied in discourse with others, or is entirely quiescent when the body is relaxed in sleep, but the respiration of air does not cease, though the will gives no co-operation to this end. Now I suppose, since the heart is surrounded by the lungs, and in the back part of its own structure is attached to them, moving that organ by its own dilatations and compressions, that the inhaling and exhaling of the air is brought about by the lungs: for as they are a lightly built and porous body, and have all their recesses opening at the base of the windpipe, when they contract and are compressed they necessarily force out by pressure the air that is left in their cavities; and, when they expand and open, draw the air, by their distention, into the void by suction.