276 



FUNCTIONS OF NUTRITION. 



FIG. 59. 



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



to the ventricles, and from the 

 ventricles to the arteries, but 

 close against its return in the 

 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. On the contraction 

 of the right ventricle, the tri- 

 cuspid valves shut back, pre- 

 venting its return into the 

 auricle (Fig. 59) ; and it is 

 driven through the pulmonary 

 artery to the lungs. Returning 

 from the lungs, it enters the left 



RIGHT CAVITIES OF THE HEART ; Auriculo-ventricu- auricle, thence passes into the 

 lar Valves closed, Arterial Valves open. Jeft yentricle) from which it ig 



delivered into the aorta, and distributed throughout the body. The two 

 streams of blood, arterial and venous, in their passage through the heart, 

 follow, in each case, a curvi- p IG 60 



linear and more or less spiral 

 direction ; the axes of the cur- 

 rents crossing each other in the 

 right and left cavities respect- 

 ively (Fig. 60). The venous 

 blood, received by the right au- 

 ricle from the venae cavse, passes 

 downward and forward into the 

 ventricle. It there turns from 

 below upward, from right to 

 left and from before backward, 

 through the conus arteriosus, to 

 the pulmonary artery. On re- 

 turning from the lungs to the 

 left auricle, it passes downward 

 into the left ventricle, when it 



makes a turn like that Upon the COURSE OF BLOOD THROUGH THE HEART. a, a. 



. |_, . -, . , , Vena cava, superior and inferior, ft. Right veil- 



Tight Side, passing from below tricle . c . p u i mon ary artery, d. Pulmonary vein. 



Upward and from left to right, Left ventricle. /. Aorta. 



behind the conus arteriosus, and crossing its axis at an acute angle, to the 

 commencement of the aorta. The aorta, though at its origin somewhat 

 posterior to the pulmonary artery, soon comes to the front in its arched 

 portion, while the pulm onary artery runs almost directly backward. Thus 

 the two blood-currents twist spirally round each other in their course. 



