» The Pleural Cavities

The arrangement of the pleura follows the usual pattern in that the thoracic interior is divided into two pleural cavities by an intermediate septum, the mediastinum. The mediastinum is weak because of poorly developed subpleural connective tissue.

The projection of the pleural cavities on the chest wall is always a matter of clinical significance. The mediastinal pleura is reflected onto the thoracic wall within the costovertebral gutter, and the costal pleura thus extends above the ventral border of the vertebral bodies. The ventral limit of the costal pleura follows an irregular line that passes over the costal cartilages. Cranially, the pleural sac extends medially to the first rib and beyond this on the right side for several centimeters into the neck (cupula pleurae), where it may be punctured by penetrating wounds that appear to spare the thorax. The caudal reflection of the costal pleura onto the diaphragm begins at the vertebral end of the 17th rib and is then deflected caudally to reach the middle of the last rib before turning forward. It then follows a more conventional course that intersects successive ribs at progressively lower levels until it continues along the eighth rib cartilage to the sternum. This line traces a slight dorsocranial concavity (Figs. 20.1/3 and 20.2/3).

FIG. 20.5 Dorsal view of the lungs and bronchial tree (schematic). 1, Apex (cranial lobe) of left lung; 2, base (caudal lobe) of left lung; 3, tracheobronchial lymph nodes.

As always, the pleural cavities are considerably larger than the lungs, even when there is maximal inflation. There thus exist potential spaces (the costomediastinal and costodiaphragmatic recesses) along the ventral and caudal margins of the lung that are never utilized and vary with phase of respiration. The costodiaphragmatic recess lies over the intrathoracic part of the abdomen and provides a potential route for the puncture of certain abdominal organs. Obviously the risk of