The two sides of the heart being quite separate, it is clear that all blood ejected by the right ventricle into the lungs must return to the left auricle, to be driven by it round the body. Yet it does not follow that the heart must at each stroke drive exactly the same quantity of blood into the pulmonary artery and into the aorta. On the average, each of the two sides ejects the same amount—about 3 ounces. Nor does it follow that as much blood is lodged in the lungs as in the whole of the rest of the body. The amount varies, but on the average the lungs contain not more than one-fifteenth of the whole blood. The heart may be likened to two cogwheels; the blood-stream to a chain, folded into a figure eight, against which the cogwheels work. Synchronously each cogwheel lifts a link, the right one of the smaller, the left one of the larger loop. Any given link returns to its starting-place in half a minute. Such an illustration gives an idea of the arrangement of the circulation as a whole, although the motion of a fluid is widely different from the motion of a chain. If, the jugular vein of the neck being cut, a colouring matter—such, for example, as ferrocyanide of sodium or methylene-blue—is injected into its central end towards the heart, it begins to appear in the blood which issues from its distal end in half a minute. In this short space of time it has passed through the right heart, through the lungs, through the left heart, and through the vessels, arteries, capillaries, and veins, of the head. Half a minute is therefore the “circulation-time.” Not that all the blood-corpuscles of the body make the circuit as rapidly as this. The time taken depends upon the particular route they follow in the greater or systemic circulation. Some traverse the vessels which supply the walls of the heart itself—a short journey; others go down to the foot and up again. But the average circulation time does not exceed a minute or a minute and a half. It is particularly in the veins of the liver and other abdominal viscera that blood tends to linger. Usually half the blood of the body, or even more, is lodged in these capacious reservoirs. It is thanks to their capacity for storing blood that a supply is provided adequate to meet any special demand. If a man runs a few hundred yards, two-thirds of the whole blood of the body is transferred to his limbs. It is quickly withdrawn from the abdominal vessels when it is wanted elsewhere; but, failing an efficient cause for removing it, its accumulation induces lethargy. Even tight-lacing has been defended by an eminent physiologist on the ground that it prevents accumulation of blood in the abdomen. But tight-lacing diminishes the capacity of the chest, hampers the action of the heart, checks the circulation, distorts the abdominal viscera, and generally deforms and jams the domestic machinery, even though the professor be right in his view that visceral compression may favour alertness of mind.