Page:The Works of William Harvey (part 1 of 2).djvu/475

 pulsion of the blood, as clearly appears in all animals furnished with red blood ; and the office of the pulsating vesicle in the generation of the chick ab ovo, as well as in the embryos of mammiferous animals, is not different, a fact which I have re- peatedly demonstrated to others, showing the vesicula pulsans as a feeble glancing spark, contracting in its action, now forc- ing out the blood which was contained in it, and again relaxing and receiving a fresh supply.

The supremacy of the blood farther appears from this, that the pulse is derived from it ; for, as there are two parts in a pulsation, viz. : distension or relaxation, and contraction, or diastole and systole, and, as distension is the prior of these two motions, it is manifest that this motion proceeds from the blood; the contraction, again, from the vesicula pulsans of the embryo in ovo, from the heart in the pullet, in virtue of its own fibres, as an instrument destined for this particular end. Certain it is, that the vesicle in question, as also the auricle of the heart at a later period, whence the pulsation begins, is excited to the motion of contraction by the distending blood. The diastole, I say, takes place from the blood swelling, as it were, in con- sequence of containing an inherent spirit, so that the opinion of Aristotle in regard to the pulsation of the heart, namely, that it takes place by a kind of ebullition, is not without some mixture of truth ; for what we witness every day in milk heated over the fire, and in beer that is brisk with fermentation, comes into play in the pulse of the heart ; in which the blood, swelling with a sort of fermentation, is alternately distended and repressed ; the same thing that takes place in the liquids mentioned through an external agent, namely adventitious heat, is effected in the blood by an intimate heat, or an innate spirit ; and this, too, is regulated in conformity with nature by the vital principle (anima), and is continued to the benefit of animated beings.

The pulse, then, is produced by a double agent : first, the blood undergoes distension or dilatation, and secondly, the vesicular membrane of the embryo in the egg, the auricles and ventricles in the extruded chick, effect the constriction. By these alternating motions associated, is the blood impelled through the whole body, and the life of animals is thereby continued.

Nor is the blood to be styled the priniigenial and principal