The first vessels into which the blood is driven by the beat of the heart are the arteries. These are tubes which conduct it from the heart to all parts of the body. Of large size where they commence, the pulmonary artery from the right ventricle and the aorta from the left ventricle, they divide and subdivide almost always at an acute angle till they terminate in the capillaries. They possess three tunics or coats. The outer one, sometimes named the adventitia, is thin, strong, and resistant, and is composed of connective tissue, with some elastic fibres; in it run the small vessels and nerves which supply the walls of the vessels themselves. The middle coat differs according; to whether a large or a small artery is under observation. In the larger arteries it is chiefly composed of elastic fibres, with a few unstriped muscular fibres interspersed amongst them. In the smaller arteries the elastic tissue becomes progressively less and less marked as they diminish in size, being replaced by the muscular tissue, which at last forms almost the whole thickness of the middle coat, the fibres for the most part running in a circular direction. The internal coat is composed of a sheet of elastic tissue with large apertures in it. It is lined by a layer of flat, endothelial cells, which are therefore in contact with the current of blood traversing the vessels. The nerves of the arteries form net-works in the substance of the vessel wall. The several coats of the arteries endow them with strength to enable them to resist the pressure of the blood, and also with elasticity and contractility. The elasticity is best marked in the large arteries, the contractility in the smaller ones. Both properties fulfil very important purposes. With each beat of the heart a pint or more of blood is suddenly injected into each of the great arteries. The shock and jar that this would produce through the entire system is almost entirely abolished by the great elasticity of the walls of the pulmonary artery and aorta. These vessels yield, and, greatly widening, receive the new column of blood with facility. But on the instant of the heart ceasing to deliver the last drop of its contents, they immediately recoil. The first effect of the recoil is to close the semilunar valves, the next to cause the blood to move onwards and distend the next part, of the artery in front. This having expanded, though to a less degree (for part of the blood in the arteries is escaping into the capillaries), now retracts on the blood within it, and as the arteries at each division present a larger area, and therefore a broader stream, whilst more and more blood is entering the great capillary sea, the interrupted current observed in the larger vessels, due to the intermittent action of the heart, is gradually converted in the smaller vessels into a uniform, steady, and continuous stream. Thus the pulse, which is very perceptible in the larger vessels, becomes imperceptible in the smaller ones. Subsidiary purposes for which the elasticity is useful are that it enables the limbs to be freely bent and stretched and otherwise moved without risk of rupture. It also enables the arteries to accommodate themselves to the considerable variations that occur in the absolute quantity of blood in the system. Lastly, the elasticity of the vessels reduces the chances of death by hemorrhage, partly by retracting the cut artery in its sheath, and partly by diminishing its calibre. The elasticity of the vessels is not a new or active force in effecting the circulation; it is passive, and represents the stored-up energy of the heart, which, during contraction, is expended in dilating the large vessels, and is given out again by them during the period of quiescence or relaxation of the heart.
Fig. 192. - Transverse Section through a small Artery and Vein.
A, Artery. V, Vein, a, Endothelial Cells with Nuclei. b, Elastic Layer of Tunica Intima. c, Tunica Media, d, Nuclei of its Muscular Fibres, e, Tunica Adventitia, showing Connective-tissue Fibres and Corpuscles.