The large arteries differ from those of medium size mainly in the fact that there is no sharp line of demarcation between the intima and the media. There is also much more elastic tissue distributed in firm bundles throughout the media, and there are fewer muscle fibers, giving a more compact appearance to the artery as seen in cross section. The predominance of elastic tissue permits of great distention by the blood forced into the artery at every heartbeat, the caliber of the tube being less markedly under the control of the vasomotor nerves than is the case in the small arteries, where the muscle tissue is relatively more developed. The adventitia of the large arteries is strong and firm, and is made up of interlacing fibroelastic tissue, of which some of the bundles are arranged longitudinally. Veins The walls of the veins are thinner than those of the arteries; they contain much less elastic and muscular tissue, and are, therefore, more flaccid and less contractile. Many veins, particularly those of the extremities, are provided with cup-like valves opening toward the heart. These valves, when closed, prevent the return of the blood to the periphery and distribute the static pressure of the blood column. The bulgings caused by the valves may be seen in the superficial veins of the arm and leg. There are no valves in the veins of the neck, where there is no necessity for such a protective mechanism, gravity sufficing to drain the venous blood from the cranial cavity. Capillaries These are endothelial tubes in the substance of the organs, the tissue of the organ giving them the necessary support. They are the final subdivisions of the blood vessels, and the vast capillary area offers the greatest amount of resistance to the blood flow, thus serving to slow the blood stream and allowing time for nutritive substances or waste products to pass from and to the blood. Usually the capillaries are arranged in the form of a network, the channels in any one tissue being of nearly uniform size, and the closeness of the mesh depending upon the organ. As far back as 1865, Stricker observed contraction of the capillaries. This observation was apparently forgotten until revived again by Krogh recently. The latter finds that the capillaries are formed of cells which are arranged in strands encircling the vessel. The capillaries are rarely longer than 1 mm., and, according to Krogh, are capable of enormous dilatation. The rate of flow through any capillary area is very inconstant, and the usual explanation has been that the capillaries were endothelial tubes the blood flow of which was dependent upon the contraction or dilatation of the terminal arterioles. The actual fact that in an observed capillary area some capillaries are empty renders the above explanation untenable. The color of a tissue depends upon the state of filling of the capillaries with blood.