Stimulated by the blood distending them, the ventricles then contract simultaneously like the auricles, only with much greater force: for the right ventricle has to drive the blood all through the vessels pervading the lungs back to the left auricle; whilst the left ventricle, which is proportionately stronger than the right, has to send its contents to the furthest extremities of the body. They then relax, in order that conditions of their internal pressure may favour another inflow from the auricles, return of blood from the arteries being, as in the preceding case, prevented by valves. The pressure in the arteries during life is always fairly high; indeed, the ventricles have to get up a considerable force before the valves leading from them will open. The result of this is not only that the blood is driven along them with a rush, but also that they are slightly distended at each beat; and so, owing to the elasticity of their walls, the blood continues to flow forwards even between the beats of the heart. The rest of the journey is quite simple; the pressure in the capillaries is lower than in the arteries, and the pressure in the veins lower than in the capillaries, and lower in the veins, too, as they approach the heart, till, where they join the auricle, it is actually minus, and the blood has no other course open to it but to return to the auricle. It looks as though accidents might happen in the veins owing to there being so low a pressure there to direct the current, but this is prevented by the presence of valves at intervals, to stop any return. The rate at which the blood travels is another point which has an important bearing on the nutrition. It does its work—_i.e._, gives out nutriment and picks up refuse—whilst flowing through the capillaries; so here one finds that it moves slowly. On the other hand, the sooner it reaches them the better, so it races fast through the arteries. Finally, its return to the heart need not be delayed, so it is quickened up again through the veins. The principle by which this variation in the rate of flow is obtained is simple and inevitable. If a tube through which liquid is flowing is not the same size all the way along, the liquid will be found to flow faster in the narrow parts than in the wider ones. Now, in branching, the arteries do not keep becoming smaller in regular proportion, and the result is that the capillaries have collectively a diameter five hundred times larger than the aorta; hence the blood flows through them only one-five-hundredth of the pace at which it leaves the heart. But in uniting again to form the veins their cross-section is reduced once more, so that that of the large veins near the heart is only two and a half times larger than that of the aorta, and hence a flow only two and a half times slower results.