THE ARTERIES. 281 



into branches which pass to the lobes of the left lung. The 

 point of division of the left branch of the pulmonary artery lies 

 craniad of all the lobes of the lung, so that the lobes of the left 

 lung are said to be all liyparterial, i.e., below (or caudad 

 of) the artery. 



The right branch passes under (dorsocaudad of) the aortic 

 arch, and reaches the right lung at about the junction of the 

 cranial lobe with the remainder of the lung. The cranial lobe 

 of the right lung is therefore said to be eparterial, since it is 

 craniad of the pulmonary artery ; the other lobes are liypar- 

 terial. The right branch divides at its entrance to the lung 

 and is distributed to its lobes. 



2. Aorta (Fig. 115, g; Fig. 118, a). 



The aorta is the single great vessel which conveys blood 

 from the left ventricle. It makes a sharp semicircular curve 

 dorsad (Fig. 1 18, a') and to the left, passes caudad at the left 

 side of the vertebral column, and passes between the crura of 

 the diaphragm to reach the abdominal cavity. It is divisible 

 into thoracic aorta (Mg. i 18) and abdominal aorta (Fig. 126). 



A. Thoracic Aorta (Fig. 118). — The first portion of the 

 thoracic aorta, curved as above described, is the aortic 

 arch (rt'). It lies in the thoracic cavity opposite the interval 

 between the third and fourth or fourth and fifth ribs. It is 

 separated from the vertebral column on the right by the 

 superior vena cava (</), and on the left by the oesophagus (4). 

 At its beginning it lies a little to the right of the median plane; 

 but it passes at once to the left side of the vertebral column. 



Branches of the Thoracic Aorta. 



I. Aa. coronariae. — The coronary arteries are two. They 

 arise from the aortic sinuses. The left one (Fig. 118,/) leaves 

 the aorta on the left side, passes dorsad of the pulmonary artery, 

 and divides into two branches, one of which follows the auricu- 

 loventricular groove (sulcus coronarius) to the dorsal side of the 

 heart and sends branches to the adjacent heart-walls, while the 

 other runs onto the ventricles, following approximately the 

 ventral border of the ventricular septum. The right coronary 

 artery passes in the auriculoventricular groove toward the right 

 and dorsad, and supplies the adjacent walls of the heart. 



