Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: into the right chamber of the heart. These veins are called the superior and inferior venae cavae. The arteries are furnished with elastic tissue which gives them elastic properties; i. e. they are distensible under force, but their elastic recoil tends to restore them to their original size. They are also furnished with circular muscles, forming part of their walls, (vaso-motor muscles), by whose contraction and relaxation the vessels can be contracted and dilated, Tyco Sphygmomanometer. Dr. Beachler's Cabinet Size Sphygmomanometer. respectively. The larger arteries are principally rich in elastic tissue; in the smaller arteries the muscular element predominates. The capillaries are neither elastic nor muscular. The veins are both, to a very slight degree, but as compared with the arteries we may regard them as almost inelastic and non- muscular. The force of the heart-beat expends itself in maintaining the pressure in the arteries; it is lost by the time the blood enters the capillaries. When the blood emerges from the capillariesinto the veins, some slight impetus is furnished it in the suction referred to above, due to the vacuum made by the dilatation of the heart chamber in diastole; but chiefly the blood is kept moving by the difference of pressure in the arteries and in the veins, and by means of the valves, all closing backward, with which the heart and the veins are supplied. We have, then as factors in the resistance offered by the vessels to the flow of blood, the following: (1) Friction between the blood-stream and the vessel walls, (2) The elastic recoil of the artery walls, (3) The contraction of the vaso-motor muscles, The net resultant of the product of these factors on the one hand, and the force of the heart beat on the other hand, con...