of dispute, cannot without effort realise what Harvey had to rid his mind of before he could see as we do. He had to get rid of a mass of time-honoured hypotheses, mainly false but not wholly so, in which an admixture of truth made the error enduring-so enduring as to influence to the present day the belief of the laity and the language of all.
In Harvey's time, as at the time of Galen, spirits were active in the human economy. There were three-the natural, the vital, and the animal. We hear of spirits even now, but they are irregular practitioners, without any recognised place in our philosophy. Up to Harvey's time spiritual essences, not implying the supernatural, but only aerial or gaseous, played a part which we now attribute to the blood in its liquidity. We can discern a sort of forecast of oxygen obscured in the old phraseology. The vital spirits were brought to perfection in the left ventricle from materials, air in particular, obtained from the lungs. As the fountain and workshop of the vital spirits,' the heart was the origin of vital heat
