17. This then is the cause of this involuntary respiration—the impossibility that the fiery element should remain at rest: for as the operation of motion is proper to heat, and we understand that the principle of heat is to be found in the heart, the continual motion going on in this organ produces the incessant inspiration and exhalation of the air through the lungs: wherefore also when the fiery element is unnaturally augmented, the breathing of those fevered subjects becomes more rapid, as though the heart were endeavouring to quench the flame implanted in it by more violent breathing.