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 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. GALEN ON THE NATURAL FACULTIES[5] BOOK I I Since feeling and voluntary motion are peculiar to animals, whilst growth and nutrition are common to plants as well, we may look on the former as effects[6] of the _soul_[7] and the latter as effects of the _nature_.[8] And if there be anyone who allows a share in soul to plants as well, and separates the two kinds of soul, naming the kind in question _vegetative_, and the other _sensory_, this person is not saying anything else, although his language is somewhat unusual. We, however, for our part, are convinced that the chief merit of language is clearness, and we know that nothing detracts so much from this as do unfamiliar terms; accordingly we employ those terms which the bulk of people are accustomed to use, and we say that animals are governed at once by their soul and by their nature, and plants by their nature alone, and that growth and nutrition are the effects of nature, not of soul. GALÊNOU PERI PHYSIKÔN DYNAMEÔN A I