CORONARY ARTERIES. 401 



belonging to different lobules do not anastomose ; from these 

 capillaries spring the radicles of the pulmonary veins. The 

 pulmonary artery at its origin is joined by its right side to the 

 aorta, round which it winds, both vessels being enclosed in a 

 serous tube, formed by a continuation of the visceral layer of the 

 pericardium ; above this is attached the fibrous layer of the same 

 membrane, which is carried onwards for a short distance as a 

 tubular sheath. About its centre, this artery is united to the 

 posterior aorta by a short, dense, fibrous cord, the remains of the 

 foetal ductus arteriosus ; its walls are thinner than those of the 

 aorta. The pulmonary is the only artery ia the adult that 

 conveys venous blood. 



COMMON AORTA. 



(Fig. 154. e.) 



The common or primitive aorta is the main trunk of the 

 arterial system. It arises from the antero-superior part of the left 

 ventricle, being attached to the aortic fibrous ring, and guarded 

 by the sigmoid valves ; it passes upwards and forwards for nearly 

 two inches, then divides into two, the anterior and posterior 

 aortce, the former supplying the fore extremity, neck, and head, 

 the latter the rest of the body, and limbs. The aorta has on its 

 right side the right auricle, and on its left the pulmonary artery, 

 to which it is joined by cellular tissue, the two lying in one 

 sheath. The trunk is traversed by the cardiac nerves. 



CORONARY ARTERIES. 



(Fig. 154. d.) 



In addition to its terminal divisions, the aorta gives off the 

 right and left coronary, or cardiac arteries, each of which arises 

 from one of the sinuses of Valsalva ; thence the right one passes 

 to the right of the pulmonary artery, under the right auricle, and 

 along the auriculo- ventricular furrow ; on reaching the right longi- 

 tudinal furrow, it divides into vertical and transverse branches. 

 The vertical descends the right longitudinal furrow, yielding 

 branches to the ventricular walls, rounds the apex, and anasto- 

 moses by convergence with the corresponding left branch ; the 

 small transverse branch passes along the transverse furrow, anasto- 

 mosing with its fellow. The left coronary artery arises as the 



2 D 



