THE VASCULAR SYSTEM. 143 



of the arch gives off two branches, the brachiocephalic 

 or innominate, and the left subclavian. Three semilunar 

 valves also guard the mouth of the aorta. The arteries 

 of the heart may easily be distinguished from the veins 

 by the fact that the walls of the former are much thicker. 



THE ARTERIES OF THE TRUNK. 



The blood-vessels form two systems of circulation; 

 the one known as the pulmonary circulation includes the 

 arteries carrying blopd from the right ventricle to the 

 lungs, and the veins returning the blood from the lungs 

 to the left auricle; the other, known as the systemic 

 circulation, includes the arteries conveying the blood 

 from the left ventricle to all parts of the body, and the 

 veins returning the blood to the right auricle. 



The main artery of the systemic circulation is the 

 aorta, which, beginning in the left ventricle, arches 

 dorsad to the heart and extends along the spinal column 

 to the tail. That portion in the thoracic cavity is termed 

 the thoracic aorta, while that portion in the abdominal 

 cavity is the abdominal aorta. Three semilunar valves 

 guard the mouth of the aorta at its opening from the 

 ventricle, in order that the blood may not be forced 

 back into the heart by the contraction of the muscular 

 coat of the arteries. 



The branches of the thoracic aorta are as follows : the 

 right and left coronary, arising from the aorta immediately 

 beyond its exit from the ventricle, are distributed to 

 the walls of the heart. From the arch of the aorta arise 

 two large branches: first, the brachiocephalic, giving 

 origin to the right subclaman supplying blood to the 

 arm, and the right and left carotids supplying the head 

 and neck; and, second, the left subclaman, supplying 



