The Pleuræ

The pleuræ are two serous sacs which line the pleural cavities and are reflected at the roots of the lungs to invest those organs. We, therefore, distinguish parietal and visceral parts of the pleuræ.

The parietal pleura (Pleura parietalis) lines the cavity in which each lung is situated (Cavum pleuræ). On the lateral thoracic wall it is adherent to the ribs and intercostal muscles and is termed the costal pleura (Pleura costalis). Behind it is closely attached to the diaphragm, forming the diaphragmatic pleura (Pleura diaphragmatica). The part which is in apposition with the opposite sac or covers the mediastinal organs is termed the mediastinal pleura (Pleura mediastinalis); where this layer is adherent to the pericardium, it is distinguished as pericardiac pleura (Pleura pericardiaca).

The pleura is reflected at the root of the lung, which it covers, constituting the visceral or pulmonary pleura (Pleura pulmonalis). Behind the root of the lung a considerable triangular area is not covered by the pleura, the two lungs being attached to each other by connective tissue in this situation.

The ligament of the lung (Lig. pulmonale) is a fold formed by the reflection of the pleura from the mediastinum and the diaphragm to the lung, behind the triangular area of adhesion just mentioned. It is seen when the base of the lung is drawn outward. It contains elastic tissue, especially in its posterior part.

The right pleura forms a special sagittal fold (Plica venæ cavæ) about a handbreadth to the right of the median plane, which incloses the posterior vena cava in its upper edge and gives off a small accessory fold for the right phrenic nerve. The fold arises from the thoracic floor and from the diaphragm below the foramen venæ cavæ, and intervenes between the mediastinal lobe and the body of the right lung. It is delicate and lace-like.

The posterior mediastinum is very delicate below the œsophagus, and usually appears fenestrated; when these apertures are present, the two pleural cavities communicate with each other.

The pleural sacs contain a serous fluid, the liquor pleuræ; in health there is only a sufficient amount to moisten the surface, but it accumulates rapidly after death.