508 TEXTBOOK OP ZOOLOGY 



arch on each side divides into a pulmonary artery to the lungs and 

 a large cutaneous artery, which passes outward to the skin. Impor- 

 tant branches of the cutaneous are: the auricularis, supplying the 

 tympanum and adjacent head region ; the dorsalis, supplying the skin 

 of the back ; and the lateralis, which is distributed to the skin of the 

 side. 



The Veins. — These vessels usually parallel the arteries that brought 

 blood to the tissues from which the veins are returning it. The walls 

 of the veins are thinner and not as elastic as those of the arteries. 

 Many veins, particularly those of the limbs, have semilunar valves 

 on the internal surface of the wall which open in the direction of 

 flow and prevent the backflow of blood. 



In returning blood to the heart, the venous system carries some of 

 the blood through the kidneys or through the liver, providing renal 

 or hepatic filters to eliminate urea and other waste products from the 

 blood or to alter it chemically. Pulmonary veins from the lungs 

 carry oxygenated blood, which differs from the type of blood found 

 in the other veins. 



The venous circulation, therefore, may be divided into four main 

 systems: the systemic, hepatic portal, renal portal, and pulmonary 

 systems. 



The systemic veins carry the greatest load of blood to the heart. 

 The larger collecting veins of the system consist of two precavals 

 receiving blood from the anterior parts of the body, except the lungs, 

 and a single postcaval or posterior vena cava receiving blood from 

 the posterior parts of the body. The two precavals empty into the 

 anterior end of the sinus venosus of the heart, and the posterior vena 

 cava empties into its posterior end. 



Each of the two anterior precavals receives blood from three 

 branches: (1) the external jugular bringing blood from the tongue, 

 hyoid, thyroid, pseudothyroid, and floor of the mouth; (2) the in- 

 nominate vein, made up of a fusion of the internal jugular returning 

 blood from the brain and other parts of the head, and the subscapular 

 vein bringing blood from the back of the arm and shoulder; and 

 (3) the subclavian vein, a fusion of the brachial vein, returning blood 

 from the forelimb, and the large musculocutaneous vein, which forms 

 an ellipse down the side of the body and extends up into the head 

 region, returning blood from the skin and outer muscles in these 

 regions. 



