The circulation of the blood, then, according to the final view of its discoverer, is maintained by a self-regulating mechanism worked by causes operating within the blood itself, the "principal part" of the body. The systolic muscular contractions of the walls of the ventricles are caused by direct mechanical stimulation (in modern language) due to diastolic distension by blood of the relaxed muscular walls of these chambers. The blood which distends the ventricles is driven forcibly into them by the auricular systole, the muscular walls of the auricles having been stimulated to contract by diastolic distention due likewise to blood. So much of Harvey's doctrine of the heart-beat, although not that of to-day, is very effective as physiology, and has advanced with modern swiftness far beyond that of his predecessors. It seems strange, therefore, even to one familiar with the movement of the Renaissance, to be swept back nearly two thousand years under Harvey's guidance to reach the underlying cause of the phenomena. According to him the distention which stimulates the right auricle to contract is produced by an expansion of the blood of the great veins, due to the innate heat. The Harveian heart-beat is caused and initiated by an Aristotelian swelling up of the hot blood. Both this expansion and the fiery central hearth at which it is produced have been expelled by Harvey from within the fully developed heart; and the primal abode of the innate heat has been transferred to the blood, with which that heat has been intimately incorporated by him. Just without the heart, moreover, Harvey has established anew the Aristotelian seething; making this the result of what we to-day may style a localized automatism of the conjoined heat and blood. He has localized this automatism of the hot blood "in the vena cava, close to the base of the heart and to the right auricle," _i.e._, close to that region at and between the mouths of the two venæ cavæ of our present terminology, where the physiology of to-day places, not within the blood but in the texture of the walls which contain it, the seat of what is prepotent in determining the rhythm of the mammalian heart-beat. Observation shows that from seemingly pulseless peripheral veins the blood continuously enters the venæ cavæ, which pulsate visibly in the region of Harvey's swelling of the blood. Yet in his lecture notes, in dealing with the significance of the thick resistent walls of the arterial vein and the aorta, he wrote: "Neither the vena cava nor the venous artery is of such construction, because they do not pulsate but, rather, are attracted."[238] On a neighboring page he had written:-- "At the same time [that] the pulse of the artery is perceived by touch, the vena cava is attracted, as it were."[239]