194 MANUAL OF ANIMAL BIOLOGY 



posterior regions through the posterior vena cava. Opening 

 into the left auricle are the pulmonary veins which bring blood 

 to the heart from the lungs. Leading from the ventricle is a ves- 

 sel with heavy muscular walls, the conus arteriosus, which 

 branches soon after leaving the heart and forms three pairs of 

 important arteries noted below. All the blood from the ventricle 

 passes out through the conus arteriosus. The heart, as noted in a 

 previous section, is enclosed by a transparent membrane, the 



PERICARDIUM. (W. f. 121.) 



The heart of the Bird and Mammal has four chambers ; a right 

 and left auricle and a right and left ventricle. The right and 

 left sides are completely separated by a tissue wall so that there is 

 no possibility of mixing the blood in the ventricles as in the Frog 

 heart. The venae cavae open directly into the right auricle. The 

 latter communicates with the right ventricle through the tricus- 

 pid valve. The right ventricle, which is larger and heavier walled 

 than the auricle, communicates through the pulmonary artery 

 with the lungs. Into the left auricle open the pulmonary veins 

 from the lungs. The opening between the left auricle and left ven- 

 tricle is guarded by the mitral valves. The cavity of the left 

 ventricle communicates through the aorta with the general sys- 

 temic circulation. The walls of the left ventricle are very mus- 

 cular. (W. fs. 121, 123.) 



Veins. The three largest veins in the Frog are the right and 

 left anterior vena cava and the single posterior vena cava, 

 all of which open into the sinus venosus. Emptying into the 

 anterior venae cavae are smaller veins which receive the blood from 

 all the anterior parts of the body except the lungs as follows : 

 Blood from the mouth region is collected by a pair of veins, the 

 external jugulars ; from the brain and other parts of the head 

 region by a pair of internal jugulars ; from the shoulders by a 

 pair of subscapulars ; from the fore limbs by a pair of brachials ; 

 and from the side of the body and certain portions of the head by 

 a pair of musculocutaneous veins. The blood from all of these 

 veins finally empties into either the right or the left anterior vena 

 cava, from which it passes into the sinus venosus. 



Posteriorly the blood is received into the posterior vena cava 

 from the liver through the hepatic veins, and from the kidneys 

 and reproductive organs through the renal veins. Emptying 

 into these vessels are those which received blood from the body 



