MAMMALIA 289 



spread around and in the tissues by capillaries, is reassembled 

 in veins. The veins are thinner -walled than the arteries, and 

 those of the legs are supplied with pocket valves to promote 

 the circulation. 



The caudal vein and the veins from the hinder limbs, the 

 iliac and the femoral from each side, combine to form the 

 posterior vena cava, which extends along the dorsal region of 

 the abdomen, receiving lumbar veins, renal and gonadial 

 veins ovarian or spermatic hepatic veins, phrenic veins, 

 and then it passes along the mediastinum to open into the 

 right auricle. The blood from the head is returned by large 

 external and small internal jugulars, right and left. These 

 vessels are joined by the right and left subclavians, and in the 

 case of the rabbit the anterior vena cava of each side passes 

 into the right auricle as a separate vessel. In most cases the 

 left vessel formed by the union of the jugulars and the sub- 

 clavian passes across and joins the right caval vein. The 

 joining vessel is called the innominate vein. The right anterior 

 vena cava receives blood by the azygos vein from the posterior 

 intercostal veins, and by the anterior intercostal vein from the 

 anterior intercostal veins. 



The veins arising from the capillaries of the abdominal 

 alimentary canal join together to form the hepatic portal 

 vein, or simply the portal vein. This vessel divides once more 

 into capillaries in the lobules of the liver, and the arterial 

 blood brought by the hepatic artery is mixed with it. The 

 blood of the liver thus derived from two sources is returned 

 to the posterior vena cava by the hepatic veins. 



In the capillaries in all regions of the body the blood escapes 

 as a fluid with leucocytes from the vessels and bathes the 

 tissues and organs. The lymph is carried in lymph-vessels, 

 which anastomose to form a highly developed system. In the 

 mesentery the lymph-vessels are especially noteworthy, for 

 there they have had to take a share in the absorption of food. 

 The emulsified fat is conveyed by them, and the lacteals, as 

 they are called, are joined in the upper part of the mesentery 

 by vessels containing lymph from the posterior part of the 

 body. A central median collecting vessel is formed in the 

 anterior region, the thoracic duct, and it discharges into the 



