284 



AMPHIBIA 



(2) The systemic arch runs obliquely around the esophagus to 

 the dorsal surface and unites with the systemic arch of the other 

 side at about the level of the anterior end of the kidneys to form the 

 dorsal aorta. 



(3) The pulmocutaneous arch is the most posterior of the three 

 aortic arches. It divides into the great cutaneous and the pul- 

 monary arteries. 



Veins. — Except the blood coming from the lungs, all of the blood 

 is returned to the heart through the three large venous trunks which 

 enter the siy2us venosus. The two anterior vena cavae are each formed 

 through the junction of three branches, the external jugular, the 

 innominate and the subclavian. The posterior vena cava is a median 

 vein which returns to the heart the blood from the liver and kidneys 

 and indirectly the blood from the other viscera and the hind limbs. 

 The three vena cavae open into the sinus venosus. The left auricle 

 receives the pulmonary veins. 



Portal Systems. — The blood from the hind limbs does not empty 

 directly into the posterior vena cava as in the higher vertebrates, 

 but it is forced to pass through a second system of capillaries before 

 reaching that vessel. A portal vein, returning blood from the capil- 

 laries of some organ, breaks up before reaching the heart into a 

 second set of capillaries within some other organ. These again 

 unite to form a vein which carries the blood to the heart. In the 

 frog there are two portal systems, the i-enal portal system leading to 

 the kidneys, while the hepatic portal system brings to the liver blood 

 from the hind limbs and from the alimentary canal. 



Action of the Heart. — The auricles contract and the oxygenated 

 blood from the left auricle, which has come in from the pulmonary 

 vein, is forced into the left side of the ventricle; and the impure 

 blood from the right auricle, received from the sinus venosus, passes 

 into the right side and middle of the ventricle. 



The blood is prevented from being mixed by being received into 

 the before-mentioned pockets or chambers in the ventricle. The 

 impure blood, nearer the opening of the truncus arteriosus, passes out 

 first and into the pulmocutaneous arches, while the pure pulmonary 

 blood from the left side is forced out at the close of the ventricular 

 contraction and passes into the carotid and systemic arches. Thus 

 the impure blood from the heart goes mainly to the lungs and skin 

 where it is purified, while the purer blood passing out towards the 

 close of the contraction is sent throughout the body. 



