Secondary involvements are more common than primary diseases on account of the relation of the pleura to the lung.

Active hyperemia is an early stage of pleuritis.

Passive hyperemia occurs in diseases in which there is interference with respiration; is most common as a result of venous stasis due to cardiac disturbances.

Hydrothorax

May result from it.

Hemorrhage

If there is extreme congestion petechias will be found. Are most marked in death from suffocation, but also occur in diseases of the blood and in hemorrhagic diatheses.

Large hemorrhages may result from traumatism, from rupture of large aneurysms, from fractured ribs, and from malignant tumors.

The blood that escapes into the pleura will not coagulate, as a rule, and is slowly absorbed if it has not become infected. Adhesions will form to some extent.

Hydrothorax is a condition in which both pleural cavities, as a rule, contain a non-inflammatory, watery, straw-colored fluid of a low specific gravity that does not undergo spontaneous coagulation. It occurs in chronic heart and kidney disease as a part of a general dropsy. The pleura is frequently opaque and lustreless and edematous. The lymph-channels are dilated and the endothelial cells may desquamate. From the pressure of the fluid the lungs are pushed backward and may be so much compressed as to interfere greatly with respiration. The lung may be even atelectatic. The effusion may occur suddenly, as in carbon monoxid poisoning, or be very chronic.

Is generally a slight effusion into the pleural cavities just before death.

Pneumothorax is the entrance of air into the pleural sacs. Is the result of accident and is almost invariably followed by infection and empyema. It results generally from the rupture of a tuberculous lesion, from gangrene or abscess of the lung, or from the rupture of an emphysematous air-vesicle. On account of adhesions the perforation seldom occurs at the apex. May be due to perforating wounds of the chest, to the rupture of an empyema into the lung, or from perforation of a gastric ulcer.

With each inspiration air escapes from the lung into the pleural sac until the pressure becomes so great as to seriously interfere with the expansion of the organ. The lung is pushed backward, is much compressed and may be airless. The opposite organ is displaced to one side, the diaphragm downward, and the intercostal spaces bulge.

The air may be absorbed, but as a rule infection occurs, giving rise to a pyopneumothorax, a combination of air and a purulent exudate.

Pleurisy or pleuritis is an inflammation of the pleura. It may be either primary or secondary, usually the later, as it most commonly occurs in the course of inflammations of the lung, as in pneumonia, tuberculosis, and gangrene. Also from involvement in inflammatory conditions of the pericardium, of the spine, the ribs or the chest wall. May be the part of a general infectious process, as acute articular rheumatism, or septicemia.

Many micro-organisms have been found, as the streptococci and staphylococci, colon bacillus, tubercle bacillus, pneu-mococcus and many others.

The involvement may be local or general and, according to the variety of exudate, fibrinous, serofibrinous, purulent, and hemorrhagic.

A single case of pleurisy may pass through all the above stages. In them all the pleura becomes hyperemic, and instead of being smooth and glistening, is rough and dull; the two layers of pleura do not glide with ease and an exudate escapes into the cavity.

In fibrinous pleurisy there is soon an exudate of fibrin forming a thin yellowish layer on the surface. It may increase in thickness and cause the pleural surfaces to adhere slightly, giving rise to the so-called "bread and butter" pleurisy. This exudate is composed of flakes and masses of fibrin containing leukocytes. The endothelium below is thickened and in places has desquamated. The sub-endothelial connective tissue is infiltrated by round cells and the vessels are congested. The "friction rub" characteristic of this condition is due to the rubbing together of the two layers of the pleura.

The exudate may be absorbed completely, but if there has been much fibrin formation adhesions of varying density result. New capillaries penetrate the fibrin masses, the fibroblastic cells proliferate, and organization takes place. These bands, although at first delicate, soon become very dense.

They may be so extensive as to cause almost complete 24 obliteration of the pleural cavity, or be present in scattered areas only. There may be areas on the surface of the pleura of marked chronic thickening.

Serofibrinous Pleuritis

In this variety there is a large amount of serous exudate as well as fibrinous. It may follow the fibrinous form, but usually begins with a serous outpouring. The fluid is denser than that in hydrothorax, and contains bits of fibrin as well as red and white blood cells in small numbers. The amount of fluid may be very little or as much as several liters. The exudate may become somewhat hemorrhagic if large numbers of erythrocytes are present. The lung is pushed backward and the neighboring organs pressed upon.

Hemorrhagic pleuritis is generally the result of tuberculosis or of malignant disease of the pleura. The exudate is chiefly serous, with red blood-cells present, but at times may be almost pure blood.

Empyema or purulent pleuritis is the result of infection by some one of many micro-organisms. It may begin as a purulent pleurisy or it may follow infection of a sero-fibrinous pleuritis. It may result from some traumatism causing an opening into the pleural sac or occur in the course of disease of the lung. In the adult its cause is most frequently the streptococcus, in children the pneumococcus. Tuberculous infection in adults is nearly twice as frequent as in children. The infecting organism, whatever it is, can be carried to the pleura either by means of the lymphatics or the blood-vessels. The organisms most commonly found are the streptococcus, pneumococcus, tubercle bacillus, staphylococcus and Bacillus pyocyaneus.

In the pleural cavity there is found a small or a large amount of a cloudy purulent fluid which contains great numbers of pus-cells. The color may be at times greenish, although usually yellowish. The pleurae are generally thickened and congested and covered with flakes of fibrin and degenerated endothelium. The pus may be completely absorbed and the two inflamed pleurae unite with dense adhesions, or it may become cheesy and undergo calcareous infiltration. The changes are most marked in the visceral pleura, which becomes greatly thickened and at first is soft and edematous, while fluid is still present. When the exudate disappears it becomes very hard and callous.

During the course of the empyema there is always more or less involvement of the lung. The fluid by its presence tends to push the lung backward and compress it. This may continue till expansion is impossible and atelectasis occurs. There may be an infection with resulting pleurogenic pneumonia. Rupture of the empyema into the lung sometimes happens, in which case the result is generally fatal.

Tuberculosis of the pleura is rare as a primary lesion; is usually secondary to similar disease of the lung or adjacent tissues. The primary form occurs as small, round, pearly bodies about the size of a pea. The pleura may be involved in the course of a general hematogenic infection. Will vary greatly in appearance; many small miliary tubercles in some cases, while in others the pleura may be covered by a wide-spread exudate. The fluid present may be sero-fibrinous, purulent, or hemorrhagic. It may become inspissated and calcification take place.

Syphilis of the pleura may be present as a part of a general syphilitic infection, but it is unimportant clinically and difficult to recognize at any time.

Tumors of the pleura are not very common, the most frequent variety being the endothelioma. It may be present as a diffuse infiltration of the pleura, resembling somewhat old adhesions, or in nodules scattered about. Secondary growths, as sarcoma or carcinoma, may result by metastasis or by direct extension from malignant disease of adjacent tissue, particularly by extension of carcinoma of the mammary gland through the chest wall. Other varieties found are the fibroma, lipoma, osteoma, and chondroma.

Parasites

Echinococcus cysts are occasionally found.