BOOK THREE

 [p. 315]previously drawn. Now Nature foresaw this necessity,

cf. p. 34, note 1.

and provided the cardiac openings of the vessels with membranous attachments,

cf. p. 121, note 4.

to prevent their contents from being carried backwards. How and in what manner this takes place will be stated in my work "On the Use of Parts," where among other things I show that it is impossible for the openings of the vessels to be closed so accurately that nothing at all can run back. Thus it is inevitable that the reflux into the venous artery

Pulmonary vein, or rather, left auricle. Galen means a reflux through the mitral orifice; the left auricle was looked on rather as the termination of the pulmonary veins than as a part of the heart. cf. p. 323, note 4. He speaks here of a kind of "physiological" mitral incompetence.

(as will also be made clear in the work mentioned) should be much greater than through the other openings. But what it is important for our present purpose to recognise is that every thing possessing a large and appreciable cavity must, when it dilates, abstract matter from all its neighbours, and, when it contracts, must squeeze matter back into them. This should all be clear from what has already been said in this treatise and from what Erasistratus and I myself have demonstrated elsewhere respecting the tendency of a vacuum to become refilled.

Horror vacui.

PART 14

And further, it has been shown in other treatises that all the arteries possess a power which derives from the heart, and by virtue of which they dilate and contract.

Put together, therefore, the two facts- that the arteries have this motion, and that everything, when