The pulmonary arteries convey venous blood from the heart to the lungs to be purified.
Veins.—The bronchial veins carry off the impure blood from the lungs.
The pulmonary veins convey the blood which has been purified by the lungs, back to the heart.
The Mediastinum.
—The mediastinum is the space left in the middle portion of the chest by the non-approximation of the two pleurae. It extends from the sternum in front to the spine behind.
Within it are the contents of the thorax, excepting the lungs. The mediastinum may be divided into two parts.
The superior mediastinum is that portion of the interpleural space which lies above the level of the pericardium. This space contains the arch of the aorta, innominate, part of the left carotid artery, part of the left subclavian artery, the upper half of the superior vena cava, the upper half of the innominate vein, the left superior intercostal vein, trachea, esophagus, thoracic duct, remains of the thymus gland, etc.
The inferior mediastinum is divided into three portions:
The anterior mediastinum is that portion in front of the pericardium. It contains nothing but some loose areolar tissue.
The posterior mediastinum is that portion back of the pericardium. It contains the descending thoracic aorta, the greater and lesser azygos veins, the esophagus, the thoracic duct, etc.