306 ELEMENTARY ZOOLOGY 



median portion of the ventricle. The one on the left side 

 is the left aortic artery or left aortic arch, while the right 

 gives off two branches. Where does the anterior one of 

 these run ? The main branch, or right aortic arcJi, passes 

 back to meet its fellow, the left aortic artery, forming with 

 it the dorsal aorta, which runs posteriorly to the end of the 

 tail. Note the various branches given off by the dorsal 

 aorta and trace some of them. Arising from the ventricles 

 beneath the two aortic arches is the pulmonary artery, 

 which goes to the lung. There the blood is purified, after 

 which it is taken up by the pulmonary vein and carried 

 back to the left auricle, whence it passes into the ventricle 

 to be mixed with the impure blood from the right auricle. 

 From the arteries the blood flows to all parts of the body 

 through fine capillaries, bathing the tissues, giving off 

 oxygen and taking up the carbonic acid gas. From these 

 capillaries it passes into veins and so back to the heart; 

 from the anterior end of the body through the jugular 

 veifo and from the posterior portion of the body through 

 the postcaval vein. Flowing forward from the tail in the 

 caudal vein, the blood enters the capillaries of the kidneys, 

 where the waste matter is taken from it. This part of the 

 circulatory system is known as then?;m/-/<?rta/circulation. 

 From the kidneys the blood flows through the postcaval 

 vein anteriorly to the heart. 



The blood which passes out from the dorsal aorta to all 

 parts of the alimentary canal is again collected into veins 

 which unite to form the mesenteric vein. This vein runs 

 to the liver, where it breaks up into capillaries. Thence 

 the blood is carried into the postcaval vein, which leads 

 directly to the heart. This part of the circulatory system 

 which collects blood from the alimentary canal and carries 

 it to the liver is called the hepatic-portal system. 



Just in front of the heart will be noted a nodular struc- 

 ture, the thyroid gland, while a little in advance of the 



