154 ELEMENTS OF MAMMALIAN ANATOMY 



Three semilunar valves also guard the mouth of the aorta, each 

 of which partly conceals an aortic sinus, sinus aortae [Valsalva?]. 

 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 carry- 

 ing blood from the right ventricle to the lungs, and the veins 

 returning the blood from the lungs to the left atrium; the other, 

 known as the systemic circulation, includes the arteries con- 

 veying the blood from the left ventricle to all parts of the 

 body, and the veins returning the blood to the right atrium. 



The pulmonary artery (A. pulmonalis) is united with the 

 aorta by a ligament, ligamentum arteriosum or ligamentum 

 Botalli, which represents the remnant of an embryonic connec- 

 tion between the two vessels, ductus Botalli (Fig. 88c). The 

 ligament is sometimes obliterated in the adult. 



The main artery of the systemic circulation is the aorta, 

 which, beginning in the left ventricle, arches dorsal 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 aortic sinus are distributed 

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

 two branches; first, the innominate (A. anonyma), giving rise 

 to a small mediastinal artery and the left and right carotids 

 supplying blood to the head and neck. The anonymous 

 artery then continues as the right subclavian to supply th^ 



