THE VASCULAR SYSTEM 285 



covered with the serous membrane which lines the pericardial cavity. 

 A similar serous epithelium is reflected on the outer side of the pericardial 

 cavity. The space between contains a small amount of fluid. 



The muscles of the myocardium are wound circularly around the 

 heart and arranged in layers. The fibers of the outer layers run at 

 approximately right angles to those of the inner layers, thus insuring a 

 maximum contraction of the heart cavities during contraction or systole. 

 The muscle of the atria is mostly independent of that of the ventricles 

 and the two are separated by a connective-tissue septum. There is, 

 however, an atrio -ventricular bundle of specialized muscle fibers which 

 extends from the atrial septum to the ventricular septum and serves to 

 convey to the ventricles the rhythm of contraction of the atria. 



The atrio-ventricular valves are attached by chordae tendineae to 

 the walls of the ventricles in such a manner as to open freely into the 

 ventricles but to prevent the return of blood when the ventricles contract. 

 The attachment of the chordae tendineae to the heart wall is by means of 

 special papillary muscles, anterior and posterior, in each ventricle. The 

 right valve is partially divided into three "cusps" and the left into two. 

 Hence they are known respectively as tricuspid and bicuspid or mitral 

 valves. (Fig. 257) 



At the opening of the aorta and of the pulmonary artery crescentic 

 semilimar valves prevent the return of blood into the ventricles. Each 

 artery contains three of these valves so arranged that under pressure of 

 the blood they meet together and occlude the lumen completely. Near 

 the semilunar valves He the openings of the coronary arteries which 

 supply blood to the wall of the heart. This blood is returned to the 

 coronary sinus of the right atrium by coronary veins which parallel the 

 coronary arteries. 



Course of Blood in the Heart. The blood from the systemic veins, 

 the precava and the postcava, enters the right atrium and by the con- 

 traction of the atrium is pumped through the tricuspid valve into the 

 right ventricle. From the right ventricle the blood is carried by the 

 pulmonary artery to the lungs, returning from the lungs by the four 

 pulmonary veins to the left atrium. Forced by the contraction of the 

 atrium through the bicuspid valve into the left ventricle, it is pumped into 

 the aorta and to all parts of the body. 



Arteries. Arteries convey blood away from the heart and, because 

 they are subjected to considerable pressure when the heart contracts, 

 their walls are correspondingly thick and elastic. A cross section shows 

 three layers, an intima, a relatively thin layer consisting of the fining 

 endotheUum and a connective-tissue layer with elastic fibers; a media, a 

 relatively thick layer of muscle and elastic fibers; and an externa, a 

 layer of loose connective tissue consisting largely of white inelastic fibers. 



