330 THE HUMAN BODY 



by its more curved border, and with its free edge turned away 

 from the heart. When the valves are in action these free edges 

 meet across the vessel and prevent blood from flowing back into 

 the ventricle. In the middle of the free border of each valve is a 

 little cartilaginous nodule, the corpus Arantii, and on each side of 

 this the edge of the valve is very thin and when it meets its neigh- 

 bor turns up against it and so secures the closure. 



The Arterial System. All the arteries of the Body arise either 

 directly or indirectly from the aorta or pulmonary artery, and the 

 great majority of them from the former vessel. The pulmonary 

 artery carries blood only to the lungs, to undergo exchanges with 

 the air in them after it has circulated through the Body generally. 



After making its arch the aorta continues back through the 

 chest, giving off many branches on its way. Piercing the dia- 

 phragm it enters the abdomen and after supplying the parts in 

 and around that cavity with branches, it ends opposite the last 

 lumbar vertebra by dividing into the right and left common iliac 

 arteries, which carry blood to the lower limbs. We have then to 

 consider the branches of the arch of the aorta, and those of the 

 descending aorta, which latter is for convenience described by 

 anatomists as consisting of the thoracic aorta, extending from the 

 end of the arch to the diaphragm, and the abdominal aorta, extend- 

 ing from the diaphragm to the final subdivision of the vessel. 



Branches of the Arch of the Aorta. From this arise first the 

 coronary arteries (crd and crs, Figs. 102 and 103) which spring 

 close to the heart, just above two of the pouches of the semilunar 

 valve, and carry blood into the substance of that organ. The 

 remaining branches of the arch are three in number, and all arise 

 from its convexity. The first is the innominate artery (Ab, Fig. 

 102), which is very short, immediately breaking up into the right 

 subclavian artery, and the right common carotid. Then comes the 

 left common carotid, Cs, and finally the left subclavian, Ssi. 



Each subclavian artery runs out to the arm on its own side and 

 after giving off a vertebral artery (which runs up the neck to the 

 head in the vertebral canal of the transverse processes of the 

 cervical vertebrae), crosses the armpit and takes there the name of 

 the axillary artery. This continues down the arm as the brachial 

 artery, which, giving off branches on its way, runs to the front of 

 the arm, and just below the elbow-joint divides into the radial 



