Inflammation of the mucous membrane surrounding the outer lung membranes (pleura), which consist of two layers separated from each other by a thin film of fluid. The inner layer is in contact with the lung, the outer layer with the wall of the chest; the film of fluid allows them to slide freely against each other. The space between the layers of membrane, the pleural cavity, is a vacuum, so they cannot drag on each other. Dry pleurisy is a condition in which the two layers adhere to each other because of the formation of fine threads of protein; this is almost always caused by a process in the lung directly under the pleura. This can be inflammation, as in tuberculosis, or lung cancer or a blood clot in the pulmonary artery. The adhesion occurs only at the site of the disorder. The condition can also occur in association with a general disease such as chronic rheumatoid arthritis. The adhesion prevents the pleura from sliding smoothly against one another, causing pain on breathing and coughing. Breathing is more shallow than usual. The further development of the condition is uncertain. There are three possibilities: usually fluid is formed, so that the dry pleurisy changes to the wet form; the threads can disappear spontaneously; or they can develop into a definitive (no longer painful) adhesion of the pleural layers. In wet pleurisy an abnormal quantity of fluid accumulates in the pleural cavity; the condition tends to be preceded by dry pleurisy. The pain disappears with the fluid. The condition is most usually caused by pneumonia or lung tumours. A small quantity of inflammation fluid causes little discomfort; larger quantities cause shortness of breath through pressure on lung tissue. The symptoms of the primary disease predominate: coughing, fever and expectoration of sputum for pneumonia; loss of weight, general malaise and coughing for malignant tumours. Pleurisy is detected by knocking and listening to the chest. If the cause is not clear a small quantity of fluid can be removed and tested; the presence of bacteria and white blood cells are an indication of inflammation, and malignant cells point to a tumour. Sometimes red blood cells are found in cases of embolism or cancer.