CIRCULATION OF THE BLOOD. Iiy 



Pericardium. The heart is surrounded by a closed fibrous membrane 

 called the pericardium. The inner surface of this membrane is lined by a 

 serous membrane, which is also reflected over the surface of the heart; 

 between the two surfaces of the serous membrane is found a small quantity 

 of fluid (the pericardial fluid), which lubricates the surfaces and prevents 

 friction during the movements of the heart. The interior of the heart is also 

 lined by a serous membrane, called the endocardium. 



Cavities of the Heart. The general cavity of the heart is sub-divided 

 by a longitudinal septum into a right and left half; each of these cavities is 

 in turn subdivided by a transverse septum into two smaller cavities, which 

 communicate with each other and are known as the auricles and ventricles, 

 the orifice between the auricle and ventricle being known as the auriculo- 

 ventricular orifice. The heait, therefore, consists of four cavities a right 

 auricle and ventricle and a left auricle and ventricle. 



Into the right auricle the two terminal trunks of the venous system the 

 superior and inferior vena caves empty the venous blood which has been 

 collected from all parts of the system; from the right ventricle arises the 

 pulmonary artery, which, passing into the lungs, distributes the blood to the 

 walls of the air-cells of the lungs; into the left auricle empty four pulmonary 

 veins, which have collected the blood from the lung capillaries; from the left 

 ventricle springs the aorta, the general trunk of the arterial system, the 

 branches of which distribute the blood to the entire system. 



The Valves of the Heart. The valves of the heart are formed by a 

 reduplication of the endocardium strengthened by connective tissue. At 

 the auriculoventricular openings on the right and left sidgs of the heart, 

 respectively, are found the tricuspid and mitral valves. The tricuspid valve 

 consists of three, the mitral of two, cusps or segments, which project into the 

 interior of the ventricle when it does not contain blood. At their bases the 

 segments are united so as to form an annular membrane attached to the 

 margin of the orifice. To the free edges of the valves are attached numerous 

 fine threads the chorda tendinece which are the tendons of the small 

 papillary muscles springing from the walls of the ventricles. 



The Semilunar Valves. At the openings of the pulmonary artery and 

 the aorta are found three cup-shaped or semilunar valves, the free edges of 

 which are directed away from the interior of the heart. The anatomic 

 arrangement of the valves is such that upon their closure regurgitation of the 

 blood is prevented. 



The Course of the Blood through the Heart. Reference to figure 17 will 

 make it clear that there is a pathway for the blood between the venae cavae 



