CIRCULATION AND RESPIRATION 131 



drained from the kidneys by the renal veins. The renal portal 

 veins and their tributaries begin in capillaries in the hindlegs and 

 end in capillaries in the kidneys. A portion, therefore, of all the 

 venous blood from the hindlegs must pass through kidneys before 

 reaching the heart. 



The hepatic portal system is made up of the hepatic portal vein, 

 which originates in the capillaries of the stomach, intestine, 

 spleen and pancreas and ends in capillaries in the lobes of the 

 liver. The anterior abdominal vein, draining parts of the hind- 

 legs, divides into two branches just before entering the liver. 

 One of these branches usually joins a branch of the hepatic portal 

 vein. Before it enters the liver, the anterior abdominal vein 

 receives a small vein, the cardiac vein, from the bulbus cordis. 



Human Heart. — The human heart consists of four chambers: 

 two atria, or auricles, and two ventricles, making possible a com- 

 pletely double circulation (Fig. 82). A sinus venosus is not 

 present in the adult. In the fish the heart cavities contain only 

 venous blood; in the frog the right atrium contains venous blood, 

 the left atrium arterial blood, while the ventricle contains both 

 kinds imperfectly separated; in the mammal the right side of the 

 heart carries only venous blood while the left side carries arterial. 

 Each atrium is connected with the ventricle of the same side by 

 an opening guarded by valves which allow the blood to pass 

 from atrium to ventricle, but not in the reverse direction. The 

 ventricles have much thicker walls than the atria and the left 

 ventricle is much stouter than the right. The inner walls of the 

 atria are relatively smooth, while those of the ventricle are raised 

 into thick muscular ridges. The atrioventricular valves are 

 thin, tough flaps, three {tricuspid) on the right and two (bicuspid) 

 on the left side of the heart. The free edges of the valves are 

 held in place by thin tendinous threads (chordae tendineae) 

 attached by thick muscular pillars to the walls (Fig. 82). The 

 contraction of the muscular pillars exerts a steady tension on the 

 tendons, keeping the valves closed during ventricular contraction. 

 Venous blood is returned from the head and anterior parts of the 

 body by a single superior vena cava that enters the right atrium; 

 from the posterior part of the body by the inferior vena cava 

 (postcava) whose entrance into the right atrium is guarded by a 

 large naplike valve (Eustachian valve). The large veins of the 

 extremities are provided with cuplike valves that allow the 



