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 hyparterial, 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 hypar- 
terial. The right branch divides dt 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 (g), and on the left by the cesophagus (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. coronaria.—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. 
