320 THE CIRCULATION. 



a position somewhat in front and above that of the left ; so that in an 

 anterior view of the heart the greater portion of the left ventricle is con- 

 cealed by the right (Fig. 100), and in a posterior view the greater por- 

 tion of the right ventricle is concealed by the left (Fig. 101) ; while in 

 both positions the apex of the heart is constituted altogether by the 

 point of the left ventricle. 



The different cavities of the heart and of the adjacent bloodvessels on 

 each side, though continuous with each other, are partially separated by 

 certain constrictions. The orifices by which they communicate are 

 known by the names of the auricular, auriculo-ventricular, and aortic 

 and pulmonary orifices ; the auricular orifices being the passages from 

 the venae eavse and 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 aorta and pulmonary artery respectively. 



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

 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 in 

 such a manner as to prevent its return in the opposite direction. The 

 course of the blood through the heart is, therefore, as follows (Fig. 102) : 



Fig. 102. 



RIGHT AURICLE AND VENTRICLE; Auriculo-ventricular Valves open, Arterial 



Valves closed. 



From the vena cava it passes into the right auricle ; and from the right 

 auricle into the right ventricle. On the contraction of the right ventri- 

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

 (Fig. 103) ; and it is thus driven through the pulmonary artery to the 

 lungs. Returning 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. The two streams of blood, 



