_Surfaces._—There are two in number. The external, costal or thoracic surface is smooth, convex and corresponds to the form of the cavity of the chest. The inner or mediastinal surface is concave, and the middle portion, where all the vessels enter and leave the lung is called the root. _Lobes._—Each lung is divided up into lobes. The right lung has three lobes, and the left lung has two lobes. _Weight._—The weight of both lungs together is about 42 ounces, the right lung being a little heavier than the left. The lungs are heavier in the male than in the female. The male lungs weigh from 42 to 45 ounces, and the female lungs weigh from 32 to 35 ounces. _Color._—The color of the lungs at birth is a pinkish white, in adult life a dark slate color, mottled in patches and as age advances this mottling assumes a black color. _Substance._—The substance of the lung is of a light porous, spongy texture. It floats in water, if it has once been filled with air. It is elastic and for this reason we always find the lung collapsed after death. The structure of the lung is such that the blood brought by the pulmonary artery comes into close relation with the air in the air-cells which enters from the bronchioles. The blood gives off carbon dioxide to the air-cells and the air in the cells furnishes oxygen for the blood. The process of respiration causes the dark blood brought from the heart by the pulmonary arteries to return to the heart as red blood in the pulmonary veins. _Arteries._—The bronchial arteries supply the lungs with nutrition. 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.