76 ESSENTIALS OF ZOOLOGY 



it leaves the body wall, is joined by the hepatic portal vein, and enters 

 the right and upper left lobes of the liver by short branches, dis- 

 charging its blood into sinusoids. 



The renal portal system, like the hepatic portal system, diverts 

 blood to a purifying organ instead of carrying it directly to the heart. 

 In this case, the blood is taken to the kidneys. 



The outer femoral vein and the medial sciatic vein collect blood 

 from the hindlegs. The femoral vein, after giving off the pelvic 

 vein, passes anteriorly and joins the sciatic, to make the renal portal 

 vein. Near the kidney this vein receives the dorsolumbar veins from 

 the body wall and, in the female, several vessels from the ovisacs. 

 The renal portal vein follows the dorsolateral margin of the kidney, 

 sending numerous transverse branches into the organ, where they 

 break up into capillaries. Blood which passes through these capil- 

 laries is purified of some of its waste products and then leaves the 

 kidney through the renal veins which empty into and form the pos- 

 terior vena cava of the systemic system. 



Pulmonary 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 com- 

 ing from the skin and bueco-pharyngeal 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 conus 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 walls tends to close the small oblique aper- 

 ture when the auricle contracts, 



