192 THE CIRCULATORY APPARATUS 



The Pulmonary Arterial System. The pulmonary 

 artery is a short, wide vessel, 2 inches in length. Com- 

 mencing at the base of the right ventricle, it curves 

 upward and backward, to end under the transverse 

 aorta by dividing into a right and a left branch, which 

 convey the blood from the right auricle to the lungs. 



This vessel, with the ascending aorta, is enclosed in a 

 sheath of pericardium. It winds around the aorta, 

 being at first in front, and later to the left side, of the 

 ascending portion. In fetal life the ductus arteriosus 

 connects it a little to the left of its division with the 

 transverse aorta. 



Each branch enters the hilum of the corresponding 

 lung; the right, the larger, passing behind the ascending 

 aorta and superior vena cava; the left, in front of the 

 descending aorta. The left divides into two branches 

 for the lobes of the left lung; the right also divides 

 into two primary branches for the upper and lower 

 lobes. From the lower one of these is sent a branch 

 to the middle lobe. The pulmonary arteries are the 

 only arteries which carry venous blood. 



The Corporeal Arterial System. The Aorta. 

 The aorta is the main trunk from which spring the 

 systemic arteries. From the base of the left ventricle 

 it runs upward, forward, and to the right as far as 

 the second right cartilage; then backward and to the 

 left, over the root of the left lung, to the fourth dorsal 

 vertebra; thence, along the spine, it descends through 

 the thorax and abdomen, to divide at the fourth 

 lumbar, into the common iliacs. 



It has been divided, for convenience of description, 

 into the arch and the descending aorta. The arch is 

 subdivided into the ascending, transverse, and descend- 

 ing parts; the descending aorta, into the thoracic and 

 abdominal portions. 



THE ARCH OF THE AORTA. The ascending part of 

 the arch runs upward, forward, and to the right, from 

 a point opposite the lower border of the third left 



