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PLEURA.

The pulmonary and costal (including the diaphragmatic) pleura consists of a layer of parallel connective-tissue fibers with but few nuclei and interspersed with numerous elastic fibers. The free surface is covered with flat, polygonal epithelium. The pleura is involved in all pulmonary inflammations that reach the pleural surface of the lung. The character of the exudate varies from a fine, macroscopically barely visible layer of fibrin to voluminous, fibrinous precipitates or purulent collections.

In fibrinous pleuritis the epithelium is lost early through swelling and fatty changes; where it is still retained it shows loosening and granular disintegration; rarely does it appear to proliferate. The fibrin, which is derived from the exudation and coagulation of plasmatic fluid, may appear first upon or under the epithelial cells. The connective tissue shows much vascular congestion, the bloodvessels often containing leukocytes or thrombi of filamentous fibrin; the lymph-vessels are widened and often contain fibrin in addition to desquamated cells. Between the connective-tissue fibers lie leukocytes, which usually also infiltrate the fibrinous deposit; the cells of the connective tissue proliferate freely and a granulation tissue is formed, composed of fibroblasts, leukocytes, and new blood-vessels, which push toward and eventually replace the fibrin. Islands of fibrin may be found inclosed in the granulation tissue; such fibrinous masses often undergo hyaline changes, and form glistening, homogeneous flakes which no longer take the fibrin stain. Gradually, the granulation tissue changes into a densely fibrillated, fibrous tissue, its cells diminish, its vessels becoming obliterated.

According to recent investigations by Ziegler, hyaline changes may also occur in mature connective tissue, probably due to an infiltration of the connective tissue with an albuminous body that has coagulated in hyaline form.

PLATE 47.

FIG. I.-Acute Fibrinous Pleuritis in Croupous Pneumonia. X66. Weigert's fibrin stain. 1, Deposit of fibrin inclosing leuko, cytes; 2, lung tissue infiltrated with pneumonic exudate; 3, thickened pleura infiltrated with young connective-tissue cells and shoots of thin walled blood-vessels, in the lumen of which are seen leukocytes and fibrin (2).

FIG. II.—Beginning Organization in Fibrinous Pleuritis. A part of the preceding figure X 340. 1, Layer of fibrin (the pleura is to be thought as near 2); 3, young blood-vessels growing toward the fibrinous layer; 4, as a result of fibrinous accumulation, one of the vessels has become thrombosed; 5, large epithelioid cells with vesicular nuclei; 6, spindle-shaped, young, connective-tissue cells; 7, lymphocytes; 8, leukocytes.

In this way arise the connective-tissue bands, wide-spread fibrous adhesions, and extensive scars in which lime-salts are later deposited.

The tuberculous pleuritis is distinguished from the simple by the presence of tubercles and caseating nodules in the granulation tissue. Otherwise, the same processes of exudation, proliferation, and organization occur.

THYROID GLAND.

The thyroid gland is a compound tubular gland whose duct, the thyroglossal,—which at one time opens at the foramen cecum on the back of the tongue, becomes obliterated in early embryonal life. The tubules are blind at both ends, and dense strands of fibrous tissue separate them into lobules. Clothed with a cubic and cylindric epithelium, which is arranged upon a basement membrane, the tubules at all stages contain a homogeneous, so-called colloid mass which also fills the lymph-vessels. The stroma is richly vascular.

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