The pace of the blood-stream must depend, obviously, on the pressure of the blood in the arteries. This pressure is altered either by changing the rapidity of the heart-beat or the diameter of the arteries, which are capable of considerable variation owing to their muscular coat. The regulation of the blood-pressure is managed by the nervous system, so does not belong here, and we may leave it after mentioning one or two facts. High pressure is due to a large quantity of blood being in the arteries, and this may be due either to the rapidity with which it is injected by the heart or to the reduced capacity of the bloodvessels themselves. High pressure, due to the latter cause, throws a great strain upon the heart, owing to the hard work it has in pumping blood into the arteries; with a low pressure the heart beats feebly, having less resistance to overcome. Blood-pressure can be raised by stimulating the muscular coat and reducing the capacity of the bloodvessels, and lowered by causing the heart to beat more slowly or by removing blood from the body. This latter operation was a favourite way with doctors of the old school; but as our knowledge of physiology, and with it our control over the vital functions, increases, such crude and heroic remedies are able to be replaced by others which are less dangerous. VI. Comparisons are rightly regarded as objectionable, so it would hardly be safe to say that the group of movements whose primary object is filling the lungs, and which we must study next, is the most important in the body, especially when we have just been speaking of the circulation, which, however, would be of but little use if the blood could not be oxidized; but we can at least say that its importance cannot be overrated, so far-reaching are its effects. The lungs are, as we have described them above, a pair of delicate membranous sacs connected by a tube, the trachea, with the alimentary canal, from which they originally budded out. They are subdivided, though how we need not describe in detail, into a vast number of small compartments, so as to give the maximum surface in the space accorded them, and the whole somewhat resembles a cluster of grapes, the stalks being the branches of the trachea. The membranous parts are pervaded by an elastic network, enveloping the compartments in such a way that it would reduce them permanently to the resemblance of a bunch of raisins rather than grapes, were it not that they are enclosed in an airtight box—the thorax—from the walls of which they cannot shrink without causing a vacuum. Owing, however, to the latter arrangement and the trachea being open to the external world, they are always more or less distended with air.