CIRCULATION 47 



pass through the aorta, anterior aorta, right axillary, cephalic, 

 and common carotid, and then through a branch of the carotid 

 to the brain, each of these arteries being a branch of the preced- 

 ing. 



The posterior aorta curves upward and backward, through the 

 diaphragm, then under the bodies of the dorsal and lumbar 

 vertebrae to the lumbo-sacral articulation. This large artery 

 supplies blood to the thoracic and abdominal organs and trunk, 

 and then terminates in four branches named iliacs, — two exter- 

 nal and two internal. 



The two external iliacs correspond somewhat to the two axil- 

 laries in front and the two femoral arteries to the two humeral, 

 for it is the femoral arteries which continue the external iliacs 

 and distribute blood to the posterior limbs and feet. 



The two internal iliacs are smaller and distribute blood to the 

 pelvic organs. 



Veins. — Veins collect and return the blood distributed by the 

 arteries and usually accompany the arteries. One large vein 

 commonly accompanies each large artery, and two small veins 

 accompany each of the smaller arteries; but this course is not 

 invariable. 



The anterior vena cava corresponds to the anterior aorta and 

 the posterior vena cava to the posterior aorta. Each returns the 

 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 between 

 voluntary muscles. 



Pulmonary veins, four to eight in number, have their origin in 

 many small branches within the lungs. They return blood from 

 lungs to heart in the pulmonary 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 at the union of the common iliac veins. It passes be- 

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

 liver and diaphragm, and discharges its blood into the right 

 auricle of the heart. 



