The pleura

The thin, healthy parietal pleura casts such a fine shadow that it is invisible radiologically; visualization of it indicates that it has been pathologically thickened. The similarity of some of the shadows cast by normal structures in the thoracic wall to pleural shadows was discussed in a previous section. For purposes of description, the pleural space is divided into costal, mediastinal, interlobar, diaphragmatic, and apical portions. The normal visceral layer of the pleura closely adherent to the lung dips into the pulmonary fissures and is usually not visible in roentgenograms, but occasionally, in apparently healthy infants and children, portions of the interlobar fissures are visualized as fine, thin lines. Anomalous fissures commonly recognized include the azygos fissure and the inferior accessory fissure. Occasionally, the caudal end of the right major fissure is observed as a curved strip of increased density ascending laterally from the right leaf of the diaphragm. Davis found that this caudal segment is seen in the films of healthy children when it is ectopic and lies forward so that part of it is in axial projection when the patient is filmed in the frontal projection. The high incidence of this ''vertical fissure line'' in association with cardiomegaly is probably due to rotation of the fissure when the heart enlarges. Friedman noted that this longitudinal pleural line can occasionally be seen bilaterally and concluded that the presence of the line indicated local underaeration of the lung