CIRCULATION, 39 



The two internal illiacs are smaller and distribute blood to 

 the pelvic organs. 



Veins. — These usually accompany arteries. One large vein 

 commonly accompanies each large artery and two small veins 

 accompany each of the smaller arteries ; but this is not a fixed law. 



The mitcrior vena cava corresponds to the anterior aorta ard 

 the posterior vena cava to the posterior aorta. Each returns tl;e 

 blood to the heart which its corresponding aorta has distributed. 



All the veins except those of the bones, small veins in the 

 feet and the veins of the brain and spinal cord, have valves. 



Valves are most common and numerous in the veins of the 

 •extremities and in those veins which pass through and among 

 voluntary muscles. 



Pulmonary veins, four in number, Have their origin within 

 the lungs. They return blood from lungs to heart in the pul- 

 monary circuit and carry pure blood. 



The anterior vena cava has its origin between the two first 

 ribs at the juncture of the two jugular and two axillary veins. 

 Jugular vein corresponds to carotid artery, and axillary vein to 

 axillary artery. 



The posterior vena cava has its origin near the last lumbar 

 vertebra by the union of the common illiac veins. It passes be- 

 neath and at one side of the bodies of the vertebrae, through liver 

 and diaphragm to the right auricle of the heart. 



LYMPHATIC SYSTEM. 



Parts. — The lymphatic system consists of vessels and glands. 

 The current in the vessels is like that in the veins in that it flows 

 towari' the heart and is very sluggish. 



Lymphatic vessels. — These have thin transparent walls and 

 are found in most of the body tissues. They have valves like 

 the veins and carry lymph, or in the lacteals, chyle. The lacteals 

 are lymphatic vessles which drain the digestive canal, chiefly the 

 small intestines. 



There are two main vessels : the thoracic and right lymphatic. 



The thoracic is situated along the vertebral column within 

 the abdomen and thorax, on the right side, near the heads of 

 the ribs. Has its origin in one dilatation or receptacle and ter- 

 :minates in another before emptying into the anterior vena cava. 



