Empedocles thinks, that the first breath the first animal drew was when the moisture in unborn infants was separated, and by that means an entrance was given to the external air into the gaping vessels, the moisture in them being evacuated. After this the natural heat, in a violent force pressing upon the external air for a passage, begets an expiration; but this heat returning to the inward parts, and the air giving way to it, causeth an inspiration. The respiration that now is arises when the blood is carried to the exterior surface, and by this fluxion drives the airy substance through the nostrils; thus in its recess it causeth expiration, but the air being again forced into those places which are emptied of blood, it causeth an inspiration. To evince which, he proposeth the instance of a water-clock which gives the account of time by the running of water. Asclepiades supposeth the lungs to be in the manner of a tunnel, and maketh the cause of breathing to be the fineness of the inward parts of the breast; for thither the outward air which is more gross hastens, but is forced backward, the breast not being capable either to receive or want it. But there being always some of the more tenuous parts of the air left, so that all of it is not exploded, to that which there remains the more ponderous external air with equal violence is forced; and this he compares to cupping-glasses. All spontaneous breathings are formed by the contracting of the smaller pores of the lungs, and to the closing up of the pipes in the neck; for these are at our command.