132 



GENERAL ZOOLOGY 



blood to flow toward the heart but not away from it (Fig. 83). 

 From the right atrium venous blood passes into the right ven- 

 tricle, whence it is pumped through the pulmonary aorta to the 

 lungs. Backflow is prevented by a set of three semilunar valves 

 located in the aorta a little beyond the ventricle. The pure, 

 aerated blood is returned from the lungs by the pulmonary veins 



LC 



LS 



PV 



RA 



TV 



Fig. 82. — Human heart dissected from the ventral side, diagrammatic, a, 

 aorta; bv, bicuspid valve; db, Ductus Botalli, a solid cord, which in the embryo 

 is a blood vessel connecting the pulmonary artery with the aorta; i, innominate 

 artery; ic, inferior cava (postcava) ; la, left auricle; lc, left carotid artery; ls, 

 left subclavian vein; pa, pulmonary aorta which divides into right and left pul- 

 monary arteries; pv, pulmonary veins; ka, right auricle; sc, superior cava; tv, tri- 

 cuspid valve. 



to the left atrium, from which it passes through the atrioven- 

 tricular aperture into the left ventricle. The latter then pumps 

 it through the systemic aorta to all parts of the body. The 

 systemic aorta, like the pulmonary aorta, contains three semi- 

 lunar valves that prevent backflow. Mammals have a hepatic 

 portal circulation, but no renal portal system, except in embryonic 

 stages. 



