PLEURA, the serous membrane lining the cavity of the thorax or chest, and which also covers the lungs. Each lung is invested by a separate pleura or portion of this membrane; the structure being spoken of rather as double pleura than as a single membrane. Like all serous membranes, each pleura is a completely closed or shut sac, and does not communicate with its companion membrane; and it further resembles the other serous membranes in that one side or part of the sac lines the containing cavity, while the other part or fold is reflected over the contained organs. In the thorax, there fore, each pleura is found to consist of a por tion lining the walls of the chest, this fold being named the pleura costalis or parietal layer of the pleura. The other fold, reflected upon the lung's surface, is named in contradistinction the pleura pulmonalis, or the visceral layer. These two folds enclose a space known as the pleural cavity, which in health contains serous fluid in just sufficient quantity to lubricate the surfaces of the pleura as they glide over one another in the movements of respiration. In front the two pleural sacs touch one another at a single point only, about the middle of the sternum, and they therefore enclose a space between them known as the mediastinum, which is again divided into anterior, micutie, posterior portions. In the mediastinal space the heart and other organs of the chest, excepting the lungs, are situated.
The pleura lines the ribs laterally, and a por tion of the sternum or breast-bone in front. From the breast-bone it passes backward over the pericardium. The pleura of the right side is wider, shorter and extends higher in the neck than that of the left side. The outer aspect of the membrane adheres to the lung-surface, and also to the pulmonary vessels as these leave the pericardium. It is also attached to the dia phragm below; elsewhere it is but loosely con nected to the contiguous surfaces. It is sup plied with blood vessels from the internal mam mary, intercostal, phrenic and other arterial trunks; its veins corresponding in nature to the arteries. Its nervous supply is derived from the phrenic and sympathetic nerves, while the absorbent or lympathic vessels are also numerous.
The diseases to which the pleura are sub ject are chiefly pleurisy or inflammation of the membrane; pneumothorax, with or without ef fusion of fluid into the pleural cavity (hydro thorax), and empyema. Hamothorax, or blood in the pleural cavity, generally results from accidental wounding of the intercostal arteries. Inflammation of the pleura of itself may ter minate in hydrothorax or empyema, the inflam matory products in this, as in other cases, ap pearing in the form of serous fluid or pus respectively. See Ptainusv.