[Illustration: FIG. 11.--A DIAGRAMMATIC VIEW OF THE BLOOD VESSELS. An artery (_a_) opens into a system of capillaries, (_c_) and after passing through these collects into a vein (_b_). Notice that the capillaries connect with other vascular territories at numerous points (_d_). If the artery (_a_) became closed the capillaries which it supplies could be filled by blood coming from other sources.] In addition to the strength and elasticity of the wall of the arteries, which enables them to resist the pressure of the blood, they have the power of varying their calibre by the contraction or expansion of their muscular walls. Many of the organs of the body function discontinuously, periods of activity alternating with comparative repose; during the period of activity a greater blood supply is demanded, and is furnished by relaxation of the muscle fibres which allows the calibre to increase, and with this the blood flow becomes greater in amount. Each part of the body regulates its supply of blood, the regulation being effected by means of nerves which control the tension of the muscle fibres. The circulation may be compared with an irrigation system in which the water supply of each particular field is regulated not by the engineer, but by an automatic device connected with the growing crop and responding to its demands. [Illustration: FIG. 12.--THE VARIOUS CELLS IN THE BLOOD. (_a_) The red blood cells, single and forming a roll by adhering to one another; (_b_) different forms of the white blood cells; those marked "1" are the most numerous and are phagocytic for bacteria.]