Chapter [XIV]
The superficial arteries, when they dilate, draw in air from the atmosphere, and the deeper ones a fine, vaporous blood from the veins and heart. Lighter matter such as air will always be drawn in preference to heavier; this is why the arteries in the food-canal draw in practically none of the nutrient matter contained in it.
Chapter [XV]
The two kinds of attraction—the mechanical attraction of dilating bellows and the “physical” (vital) attraction by living tissue of nutrient matter which is specifically allied or appropriate to it. The former kind—that resulting from horror vacui—acts primarily on light matter, whereas vital attraction has no essential concern with such mechanical factors. A hollow organ exercises, by virtue of its cavity, the former kind of attraction, and by virtue of the living tissue of its walls, the second kind. Application of this to question of contents of arteries; anastomoses of arteries and veins. Foramina in interventricular septum of heart, allowing some blood to pass from right to left ventricle. Large size of aorta probably due to fact that it not merely carries the pneuma received from the lungs, but also some of the blood which percolates through septum from right ventricle. Thus arteries carry not merely pneuma, but also some light vaporous blood, which certain parts need more [Pg lv] than the ordinary thick blood of the veins. The organic parts must have their blood-supply sufficiently near to allow them to absorb it; comparison with an irrigation system in a garden. Details of the process of nutrition in the ultimate specific tissues; some are nourished from the blood directly; in others a series of intermediate stages must precede complete assimilation; for example, marrow is an intermediate stage between blood and bone.
From the generalisations arrived at in the present work we can deduce the explanation of all kinds of particular phenomena; an instance is given, showing the co-operation of various factors previously discussed.