THE HEART. 269 



pulmonary veins into the right and left auricles; the auriculo- 

 ventricular orifices leading from the auricles into the ventricles; 

 and the aortic and pulmonary orifices leading from the ventricles 

 into the aortic and pulmonary arteries respectively. 



The auriculo- ventricular, aortic, and pulmonary orifices are fur- 

 nished with valves, which allow the blood to pass readily from the 

 auricles to the ventricles, and from the ventricles to the arteries, 

 but shut back, with the contractions of the organ, so as to prevent 

 its return in an opposite direction. The course of the blood 

 through the heart is, therefore, as follows. From the vena cava it 

 passes into the right auricle ; and from the right auricle into the 

 right ventricle. (Fig. 81.) On the contraction of the right ventricle, 

 the tricuspid valves shut back, preventing its return into the auricle 

 (Fig. 82); and it is thus driven through the pulmonary artery to the 



Fig. 82. 



KIOHT A CHICLE AXD VENTRICLE; Attricalo-veatricular Valves closed, Arterial Valves open. 



lungs. Eeturning from the lungs, it enters the left auricle, thence 

 passes into the left ventricle, from which it is finally delivered into 

 the aorta, and distributed throughout the body. (Fig. S3.) This 

 movement of the blood, however, through the cardiac cavities, is 

 not a continuous and steady flow, but is accomplished by alternate 

 contractions and relaxations of the muscular parietes of the heart 

 so that with every impulse, successive portions of blood are received 

 by the auricles, delivered into the ventricles, and by them dis- 



