1 68 THE CIRCULATION OF THE BLOOD 



valve, between the right auricle and right ventricle, figure 135, and the semi- 

 lunar valve of the pulmonary artery, the mitral valve between the left auricle 

 and ventricle, and semilunar valve of the aorta. The bases of the tricuspid, 

 figure 152, and mitral valves are attached to the walls of the venous orifices 

 respectively. Their ventricular surfaces and borders are fastened by slender 

 tendinous fibers, the chorda tendinece, to the internal surface of the walls 

 of the ventricles at points which project into the ventricular cavity in the form 

 of bundles or columns, the musculi papillares. 



The semilunar valves guard the orifices of the pulmonary artery and of 

 the aorta. They are nearly alike on both sides of the heart, but the aortic 

 valve is altogether thicker. Each valve consists of three parts which are 

 of similunar shape, the convex margin of each being attached to a fibrous 

 ring at the place of junction of the artery to the ventricle, and the concave 

 or nearly straight border being free, so as to form a little pouch like a pocket, 

 figure 151. In the center of each free edge of the valves which contains 

 a fine cord of fibrous tissue is a small fibrous nodule, the corpus Arantii of 

 the valves. 



The Arteries. The arterial system begins at the left ventricle in a 

 single large trunk, the aorta, which, almost immediately after its origin, 

 gives off in the thorax three large branches for the supply of the head, neck, 

 and upper extremities; it then traverses the thorax and abdomen, giving 

 off branches, some large and some small, for the supply of the various organs 

 and tissues it passes on its way. In the abdomen it divides into two chief 

 branches. The arterial branches, wherever given off, divide and subdivide 

 until the caliber of each subdivision becomes very minute. These smallest 

 arteries are called arterioles. These arterioles are continuous with the capil- 

 laries. Arteries frequently communicate or anastomose with other arteries. 

 The arterial branches are usually given off at an acute angle, and the areas 

 of the branches of an artery generally exceed that of the parent trunk, and, 

 as the distance from the origin is increased, the area of the combined branches 

 is increased also. As regards the arterial system of the lungs, the pulmonary 

 artery and its subdivisions, they are distributed in much the same manner 

 as the arteries belonging to the general systemic circulation. 



The walls of the arteries are composed of three principal coats, the ex- 

 ernal, or tunica adventitia, the middle, or tunica media, and the internal, or 

 unica intima. The external coat, figures 143 and 144, a, the strongest and 

 toughest part of the wall of the artery, is formed of areolar tissue, with which 

 is mingled throughout a network of elastic fibers. The middle coat, figure 

 144, m, is composed of both muscular and elastic fibers with a certain pro- 

 portion or areolar tissue. In the larger arteries, its thickness is compara- 

 tively as well as absolutely much greater than in the small arteries, consti- 

 tuting, as it does, the greater part of the arterial wall. The muscular 

 fibers are unstriped, figure 145, and are arranged, for the most part, trans- 



