52 HUMAN PHYSIOLOGY. 



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 cupped-shaped or semilunar valves, the free edges 

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

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

 the blood is prevented. 



Movement of the Blood. The blood within the vascular apparatus is 

 in continual movement from the left side of the heart through the arterial sys- 

 tem, capillaries and veins to the right side, and from the right side through 

 the pulmonary artery, capillaries and veins to the original point of departure. 

 The cause of this movement is the difference of pressure which exists 

 between the blood within the aorta and the terminations of the venae cavae, 

 and between the blood of the pulmonary artery and the pulmonary veins. 



The function of the heart is to propel the blood through the blood-ves- 

 sels, which it does by raising or maintaining this higher pressure in the 

 aorta and pulmonary artery. This is accomplished by alternate contrac- 

 tions and relaxations of its muscular walls. These two movements are 

 known respectively as the systole and the diastole. 



Course of the Blood through the Heart. The venous blood re- 

 turned to the heart by the superior and inferior venae cavse is emptied during 

 the diastole into the right auricle, in the contraction of which it is forced 

 through the right auriculo-ventricular opening into the right ventricle and 

 distends it. Upon the contraction of the ventricle the blood is propelled 

 through the pulmonary artery into the lungs, where it undergoes aeration 

 and is changed in color. 



The arterial blood is now collected by the pulmonary veins and poured 

 into the left auricle ; thence it passes into the left ventricle, which becomes 

 fully distended. Upon the contraction of the ventricle, the blood is pro- 

 pelled into the aorta, and by it distributed to the system at large, to be again 

 returned to the heart by the veins. 



Regurgitation from the ventricles into the auricles during the systole is 

 prevented by the closure of the tricuspid and mitral valves; regurgitation 

 from the pulmonary artery and aorta into the ventricles during the diastole 

 is prevented by the closure of the semilunar valves. 



