Many schemata have been devised to illustrate the vascular system; but all are misleading, inasmuch as they fail to give any idea of the extent to which the subdivision of its vessels is carried. If the water-pipes supplying a town branched until the original conduit was represented by five to six thousand million little pipes, the friction which the pumping-station would have to overcome would be very great. But little force would remain in the water when it reached the smallest pipe. Still greater is the resistance to the flow of blood, which is slightly viscous, and contains solid corpuscles, which increase friction. Two thousand miles of capillary tubing in the body of a man, without reckoning the vessels of his liver and lungs! [Illustration: FIG. 12.—A PORTION OF THE WALL OF A SMALL ARTERY CUT TRANSVERSELY AND HIGHLY MAGNIFIED. Its inner coat consists of a lining sheet of epithelial scales supported by connective tissue and a strong elastic membrane. This membrane is perforated with holes which place the lymph-spaces on its two sides in continuity. The middle coat is composed of plain muscle-fibres and patches of elastic membrane; the outer coat of elastic fibres, mostly longitudinal, and connective tissue.] Water is supplied to houses in rigid tubes. Arteries are elastic, and their elasticity is self-regulating. The cause of this will be apparent if a section of an artery is examined. It contains much elastic tissue. It also contains plain muscle-fibres. The smaller the artery, the greater is the amount of muscle relatively to the other constituents of its wall. The wall of a vein contains very little muscle, and not much elastic tissue. The muscle of all arterial walls is in a chronic state of tone. To some extent the degree of tone is varied automatically. Pressure within an artery acts as a stimulus to the muscle-fibres of its wall. Any increase leads the fibres to contract more strongly. Any diminution induces them to relax. The arteries resist distension; they do not narrow to any great extent when pressure falls. But more important than this automatic mechanism for maintaining a uniform pressure in the capillaries in general are the changes of pressure in particular localities, brought about by the mediation of vaso-constrictor and vaso-dilator nerves. In almost all organs and parts of the body the automatic tone of arteries is enhanced by impulses which flow continuously down vaso-constrictor nerves. These impulses start from, or, to speak more accurately, pass through, the vaso-motor centre in the medulla oblongata. From every part of the body impulses ascend to this centre, urging it to keep up the blood-pressure by universal constriction. Yet no separate organ would be interested in sending such a message if it were not open to it to ask at the same time that the constriction of its own vessels might be relaxed.