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present, and oftentimes the fibrinous exudate extends into the bronchi themselves, from which it may also start. The variability of the exudate does not depend so much upon the kind of micro-organism present as upon their number. The micro-organisms that are found are the diplococcus pneumonia (Fränkel-Weichselbaum), the streptococcus pyogenes, the pyogenic staphylococci, the pneumobacillus of Friedländer, the diphtheria bacillus, the influenza bacillus, and the tubercle bacillus which is found in the bronchopneumonia around tuberculous

areas.

Generally speaking, it may be said that the amount of fibrin found is in inverse proportion to the number of bacteria present; in other words, the greater the number of bacteria, the smaller is the amount of fibrin. The whole process really depends upon the chemotactic action of bacteria on the leukocytes. (See General Part, Inflammation.)

Under peculiar circumstances there are formed in the exudate of the alveoli a larger or smaller number of giant cells, especially in cases of postdiphtheric pneumonia and in cases following measles. They are seen in the alveoli as massive, polynuclear, occasionally extraordinarily largesized, polygonal, or irregularly pointed cells that inclose within their cytoplasm a number of white blood-corpuscles, nuclear and cellular fragments, and clumps of fibrin. (Plate 40, Fig. II.) These cells are formed by the conglutination of a number of detached, alveolar, epithelial cells, as well as by the multiplication of the nucleus in the cell without subsequent division of the cell-body. Occasionally, they are found in fibrous indurated lungs, resulting from the "atypically proliferated" epithelium.

The embolic and pleurogenous forms of pneumonia have been mentioned. The embolic forms are often accompanied with the formation of infarction; this occurs when bacteria alone do not reach the lung, but are car

PLATE 40.

FIG. I.-Purulent (Lobular) Pneumonia Following Diphtheria. 250. 1, Bronchus filled with pus-corpuscles and loosened epithelium, the latter in form of rows. The wall of the bronchus is infiltrated with leukocytes (pus-cells). Alveolar septa contain greatly injected blood-vessels. The alveoli are filled with a purely cellular exudate, composed largely of leukocytes and partly of desquamated epithelium.

FIG. II.-Postdiphtheric Lobular Pneumonia. X 280. Weigert's fibrin stain. The exudate in the alveoli is partly cellular and partly fibrinous (2); capillaries of the alveolar walls are greatly injected; the cellular material in the alveoli consists largely of leukocytes, less so of alveolar, epithelial, and giant cells (1). The giant cells are the result of the melting together of a number of the alveolar epithelial cells; their protoplasmic bodies are filled with various kinds of detritus, nuclear fragments, and particles of fibrin, etc.

The

ried there by an embolus in which they are inclosed. most frequent points of origin are infectious thrombi in veins in various parts of the body: for instance, the uterine veins in puerperal septicopyemia; more rarely, endocarditic vegetations in the right side of the heart. First, there is formed a hemorrhagic infarct in the affected area that soon becomes infiltrated with leukocytes as a result of the peculiar action of the micro-organisms, and more or less purulent softening of the surrounding tissue (embolic abscess) soon takes place. The picture of an insular pneumonia in this instance is of brief duration, as the process of softening of larger areas is soon accomplished. Sometimes the clump-like masses of cocci are visible with the low power.

Tuberculosis.

The histologic appearance of pulmonary tuberculosis is extremely varying and polymorphous. Formerly, before the discovery of the tubercle germ led to the establishment

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