(Chapter 5) Footnote 45:

nativum calorem (called insitus calor in Vesalius’ Preface), Galen’s e)/mfuton qermo/n, the immortal substance of life endowed with intelligence and nourished by vital pneuma. Taken from Hippocratic and Aristotelian theory, it is a fundamental concept in Galen’s physiology. See May 1968, 50ff. Though in Galen it resides chiefly in the left ventricle of the heart (where it is most intense) and the arteries, the present passage implies that the arteries carry it off from the limbs when it becomes too great.