474 



OF THE CIRCULATION OF THE BLOOD. 



by the elasticity of its substance ; for it was observed in this case to be so great, 

 that the right auricle seemed ready to burst, so great was its distension, and so 

 thin were its walls. Moreover, the large veins near the heart contract simulta- 

 neously with the auricle, and not whilst it is dilating ; so that they can have no 

 influence in causing its distension. 



503. The course of the circulating fluid through the Heart, and the action of 

 its different valves, will now be briefly described. The Venous blood, which is 

 returned by the ascending and descending Vena Cava, enters the right Auricle 

 during its diastole; part of it flows on, as already mentioned, into the right 

 Ventricle during the earlier portion of its diastole; but the Auricle, being filled 

 before the Ventricle, then contracts, and discharges its contents through the 



Fig. 139. 



The Anatomy of the Heart : 1, the right auricle ; 2, the entrance of the superior vena cava ; 3, the entrance 

 of the inferior cava ; 4, the opening of the coronary vein, half closed hy the coronary valve ; 5, the Eustachian 

 valve ; 6, the fossa ovalis, surrounded by the annulus ovalis ; 7, the tuherculum Loweri ; 8, the musculi pecti- 

 nati in the appendix auriculae; 9, the auriculo-ventricular opening ; 10, the cavity of the right ventricle ; 11, 

 the tricuspid valve, attached by the chordae tendineae to the carneae columnas (12) ; 13, the pulmonary artery, 

 guarded at its commencement by three semilunar valves ; 14, the right pulmonary artery, passing beneath 

 the arch and behind the ascending aorta ; 15, the left pulmonary artery, crossing in front of the descending 

 aorta ; *, the remains of the ductus arteriosus, acting as a ligament between the pulmonary artery and arch of 

 the aorta ; the arrows mark the course of the venous blood through the right side of the heart ; entering the 

 auricle by the superior and inferior cava, it passes through the auriculo-ventricular opening into the ventricle, 

 and thence through the pulmonary artery to the lungs ; 16, the left auricle ; 17, the openings of the four pul- 

 monary veins ; 1&, the auriculo-ventricular opening ; 19, the left ventricle ; 20, the mitral valve, attached by 

 ita chordae tendineae to two large columnae carneae, which project from the walls of the ventricle ; 21, the 

 commencement and course of the ascending aorta behind the pulmonary artery, marked by an arrow ; the 

 entrance of the vessel is guarded by three semilunar valves; 22, the arch of the aorta. The comparative 

 thickness of the two ventricles is shown in the diagram. The course of the arterial blood through the left 

 side of the heart is marked by arrows. The blood is brought from the lungs by the four pulmonary veins 

 into the left auricle, and passes through the auriculo-ventricular opening into the left ventricle, whence it is 

 conveyed by the aorta to every part of the body. 



tricuspid valves into the Ventricle, which it thus completely distends. The re- 

 flux of blood into the veins during the auricular systole is impeded by the con- 

 traction of their own walls, and by the valves with which they are furnished ; 

 but these valves are so formed, as not to close accurately, especially when the 

 tubes are distended; so that a small amount of reflux usually takes place, and 

 this is much increased when there is any obstruction to the pulmonary circula- 

 tion. Whilst the right Ventricle is contracting upon the blood that has entered 

 it, the carneas columnse, which contract simultaneously with its proper walls, 

 put the chordse tendineae upon the stretch ; and these draw the flaps of the Tri- 

 cuspid valve into the auriculo-ventricular axis. The blood then getting behind 



