The _adventitia_ is composed of loose connective tissue with some elastic fibers. A few smooth muscle cells are present, which, as are also the elastic fibers, are arranged longitudinally. The adventitia, blending with the connective tissue surrounding the arteries, serves to anchor the vessels to the surrounding structure. =Structure of the Veins.=—In many respects the walls of the veins resemble those of the arteries. The same three coats exist and the same elements enter into their structure. The transition from capillary to small veins, and from those to larger veins, is similar to the transition from the arteries to capillaries, in inverse order. The walls are not so thick as those of arteries. The elastic tissue is much less in quantity and in smaller veins disappears. There is not a clear line of demarcation between the intima and the media. The veins of birds differ from those of mammals in that they have fewer _valves_. The valves are also less perfect, and often permit a backward flow of blood. The walls of the arteries and of the veins are supplied with nutrient blood-vessels. These are called the _vasa vasorum_, or blood-vessels of the blood-vessel wall. They are mostly in the adventitia. They may arise from the vessel to which they are distributed or take origin from an adjacent vessel. These small arteries supplying the vessel coat after terminating into capillaries form small veins through which the blood, from the structure of the vessel wall, is returned. The walls of the blood-vessels are supplied with both medullated and non-medullated nerves. The non-medullated nerve fibers are axones of the sympathetic neurones and control the caliber of the vessels. These fibers are called the _vasomotor nerves_. They form plexuses in the adventitia, from which are given off branches which penetrate the media and terminate on the muscle cells. The medullated nerves are the axones of the spinal nerves. The larger fibers are found in the connective tissue outside the adventitia and give off branches to the media where they divide repeatedly, lose their sheath, and terminate in the media and at times in the intima. THE ARTERIAL TRUNKS