273. All the arteries of the body proceed from the two trunks already described; that connected with the pulmonic circle, the pulmonary artery, and that connected with the systemic circle, the aorta. These vessels, as they go out from the heart and proceed to their ultimate termination, are arborescent, that is, they successively increase in number and diminish in size, like the branches of a tree going off from the trunk (fig. CXIX. 1, 2, 3). Each trunk usually ends by dividing into two or more branches (fig. CXIX. 1, 2), the combined area of which is always greater than that of the trunk from which they spring, in the proportion of about one and a half to one. As the branch proceeds to its ultimate termination it divides and subdivides, until at length the vessel becomes so minute, that it can no longer be distinguished by the eye. These ultimate branches are called capillary vessels, from their hair-like smallness (fig. CXIX. 4); but this term does not adequately express their minuteness. It has been stated (234) that the red particle of the blood, at the medium calculation, is not more than the three-thousandth part of an inch in diameter; yet vast numbers of the capillary vessels are so small that they are incapable of admitting one of these particles, and receive only the colourless portion of the blood. [Illustration: Fig. CXIX. View of the manner in which an artery divides and subdivides into its ultimate branches. 1. Trunk of the artery; 2. large branches into which it subdivides; 3. small branches, successively becoming smaller and smaller until they terminate in 4. the capillary branches.] 274. Every portion of an artery, by reason of the elasticity of its coats, preserves nearly a cylindrical form, and as the area of the branches is greater than that of the trunks, the blood, in proceeding from the heart to the capillaries, though passing through a series of descending cylinders, is really flowing through an enlarging space. 275. The disposition of the veins, like that of the arteries, is arborescent, but in an inverse order; for the course of the veins is from capillary vessels to visible branches, and from visible branches to large trunks (fig. CXX. 1, 2, 3). In every part of the body where the capillary arteries terminate the capillary veins begin, and the branches uniting to form trunks, and the small to form large trunks, and the trunks always advancing towards the heart, and always increasing in magnitude as they approach it, form at length the two veins which it has been stated (258) return all the blood of the body to the right auricle of the heart. [Illustration: Fig. CXX.