510 TEXTBOOK OP ZOOLOGY 



wall and, in the female, several vessels from the ovisacs (uteri). The 

 renal portal vein follows the dorsolateral margin of the kidney, send- 

 ing numerous transverse branches into the organ, where they break 

 up into capillaries. Blood which passes through these capillaries is 

 purified of some of its waste products and then leaves the kidney 

 through the renal veins which empty into and originate the poste- 

 rior vena cava of the systemic system. 



Pulmonary Veins.— These veins run along the inner walls of each 

 lung, returning the oxygenated blood to the heart. The right and left 

 pulmonary veins unite to form a single vessel which empties into the 

 left auricle on its dorsal side. Other veins which take on oxygen are 

 those coming from the skin and buccopharyngeal cavity. 



The Heart. — The heart is enclosed in the pericardial cavity, which 

 is lined by a transparent tissue, the pericardium, and is separated 

 from the remainder of the body by the transverse septum. It is the 

 rhythmically contracting organ that circulates the blood. It is coni- 

 cal in shape and in the frog consists of a right and left thin-walled 

 auricle above a single thick-walled ventricle. On the ventral side 

 is a muscular tube, the C07ms arteriosus, described with the arteries. 

 It conducts blood away from the heart. On the dorsal side of the 

 heart is a thin-walled sac, triangular in shape, the sinus venosus, 

 which receives venous blood from the systemic veins. 



The sinus venosus empties into the right auricle through the sinu- 

 auricular aperture. This aperture has liplike valves on each side to 

 prevent the blood from flowing back into the sinus when the auricle 

 contracts. The smaller left auricle receives oxygenated blood from 

 the pulmonary vein. Valves are not necessary at this opening, for 

 pressure on the auricular w^alls tends to close the small oblique aper- 

 ture when the auricle contracts. 



Both auricles pass blood into the ventricle through a common open- 

 ing, the auriculoventricular aperture, which is divided by the inter- 

 auricular septum separating the two auricles. This aperture has two 

 large valves on each side and two small valves at each end which 

 regulate the discharge of blood into the ventricle and prevent its 

 backflow. 



Blood leaves the ventricle and enters the arterial system through 

 the conus arteriosus. The opening into the conus is protected by 

 three pocketlike semilunar valves which open inwardly into the conus 

 when blood is passing out but are tightly closed at other times. The 



