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 hyparterial. 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. 118, _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= (Fig. 118) 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= (_a′_). 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 (_q_), and on the left by the œsophagus (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. 1. =Aa. coronariæ.=--The coronary arteries are two. They arise from the aortic sinuses. The left one (Fig. 118, _p_) leaves the aorta on the left side, passes dorsad of the pulmonary artery, and divides into two branches, one of which follows the auriculoventricular 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. 2. =A. anonyma= (Fig. 115, _j_; Fig. 118, _b_).--The innominate artery passes craniad from the convexity of the aortic arch (_a′_). It gives rise first to a small =mediastinal artery= (_m_) which passes ventrad into the mediastinum, then to the =left common carotid= (Fig. 115, _l_), then to the =right common carotid= (Fig. 115, _m_). Sometimes the carotids are given off from a common trunk. Beyond the right common carotid the innominate is continued as the =right subclavian= (Fig. 115, _n_). [Illustration: FIG. 118.--VESSELS OF THE THORAX, VIEWED FROM THE LEFT SIDE.