In mammals, the sinus venosus is absorbed into the wall of the right atrium and the blood vessels which had emptied into the sinus (the superior and inferior vena cava and the coronary sinus) empty into the right atrium instead.  The embryonic sinus venosus is present in adult mammals only as the SA node (Kardong,461,  475).  The valves of the sinus venosus become the valve of the inferior vena cava and the valve of the coronary sinus in the human fetal heart (Torrey, 1979, p. 429-30).  Edentates retain vestiges of the sinus valves and a thin sinus venarum, a nonfunctional vestige of the sinus venosus (Webster, 1974, p. 284).  Mammalian hearts also possess an AV node in their conduction system (Kardong).  Mammalian cardiac muscle possesses T-tubules not present in the cardiac muscle cells of reptiles and amphibians (Webster, 1974, p. 179).

     While birds and mammals may share traits such as a four chambered heart, these traits arose independently in evolution.  In mammals, the interventricular septum is formed by the fusing of muscular ridges and their endocardia while in birds it is formed by the fusion of muscular trabeculae with little input from the endocardium.  The partial interventricular septum in lungfish is actually formed by a combination of these two mechanisms.  Mammals and birds also differ the manner in which interatrial openings close at birth.  In addition, birds still retain a portion of the sinus venosus separated from the right atrium by a sulcus in addition to the internal valves of the sinus venosus (Torrey, 1979). 

     In placental mammals, the ductus arteriosus is functional in the embryo but becomes the ligamentum arteriosum in adults. (Kardong 479)  A conus arteriosus exists in edentates, dogs, porpoises, and bats.  Bats retain a muscular conus which contracts as a separate chamber (Webster, 1974).  Some mammals have a septal artery to service the interventricular septum (Weichert, 1970).

OPOSSUM

CAT

GOAT

COW

SHEEP

PIG

MONKEY HEART

HUMAN HEART MODEL