Thorax and Lungs:
The lungs, a pair of conical-shaped organs lie in the thoracic cavity, protected by the bony thorax composed of the sternum and ribs a interiorly and ribs, scapulae and the vertebral column posteriorly. The apices of lungs lie just above the clavicles and the concave base posteriorly extends to the 11th or 12th rib. The lung is described as having a medistinal and a costal surface. The costal surface is in contact with the wall of the pleural cavity which consist of the ribs and intercostals muscles forming the thoracic cavity. The thoracic cavity is lined with pleura. The pleura is a continuous serous membrane; one surface of it lines the inside of the rib &ge knows as the parietal pleura and the other surface covers the lungs knows as the visceral pleura. The space between the two surfaces is known as potential space, containing a few milliliters of serous fluid that prevents friction between the two surfaces.
The lungs are separated from each other by the mediastinum which contains the heart and great vessels, the esophagus and, in its upper part the trachea. The chief feature of the mediastinal surface is the presence of the hilum where the main bronchus and pulmonary artery enter and pulmonary veins leave the lungs. Group of lymph glands which drain the lungs are situated at the hylum and at the bifurcation of trachea. The right and left lungs are divided by deep fissures into lobes, the right lungs has upper, middle and lower lobes and left lung has upper and lower. Each lobe is further divided into segments. Air is conducted to each segment through the bronchioles the smallest branches of the bronchi.