280 VERTEBRATE ZOOLOGY 



lymph vessels. The heart is a muscular pump containing two 

 auricles and two ventricles ; it drives the blood through two sys- 

 tems of arteries : ( i ) the pulmonary arteries, which take venous 

 blood from the right ventricle to the lungs to be aerated, and 

 ( 2 ) the systemic arteries, which take arterial blood from the left 

 ventricle to the various tissues and organs of the body. The heart 

 receives blood through two systems of veins: (i) the pulmonary 

 veins, which bring arterial blood from the lungs to the left auricle, 

 and (2) the systemic veins, which bring venous blood from the 

 tissues and organs of the body to the right auricle. 



The systemic veins, in turn, may be divided into two systems : 

 (1) the caval system, by which the blood is brought directly to 

 the heart, and (2) the portal system, by which the blood is taken 

 from the digestive tract and the spleen directly to the liver, from 

 which organ it goes to the heart through the hepatic veins. 



The Portal System. This system consists of the portal vein and 

 several smaller veins which meet and form this vein. The portal 

 vein and the superior, or anterior, mesenteric, the largest of its trib- 

 utaries, have just been observed. Note carefully the branches of 

 these two veins, which lie in the mesenteries. The most important 

 of these are the following : the gastrosplenic vein, a large vein which 

 meets the portal on its left side near the pyloric end of the stomach 

 and is formed by the union of numerous veins from the stomach, 

 the spleen, and the great omentum ; the duodenal vein, which joins 

 the portal posterior to the gastrosplenic, lying in the mesentery 

 of the duodenal loop and collecting branches from the duodenum 

 and pancreas ; the inferior, or posterior, mesenteric vein, which 

 joins the portal on the side opposite to the duodenal and receives 

 numerous branches from the large intestines ; the intestinal vein, 

 which joins the superior mesenteric, being formed by numerous 

 branches from the greater part of the small intestines; and the 

 ileocaecocolic vein, a large vein formed of branches from the ileum 

 and large intestines and itself a branch of the superior mesenteric. 



Trace these veins and their branches as far as possible, cutting 

 the mesenteries and separating the organs where necessary to do 

 so. Note carefully the arteries which accompany most of these 

 veins; they are branches of the mesenteric arteries. 



