THE RESPIRATORY SYSTEM. 493 



consolidated arid less dense and hard than in vigorous adults. When 

 lobar pneumonia occurs in lungs already the seat of chronic lesions, such 

 as chronic congestion, emphysema, interstitial pneumonia, or tuberculo- 

 sis, the gross appearances of the organs maydiffer in various ways from 

 those of uncomplicated pneumonia. 



FIG. 274. "ORGANIZING" PNEUMONIA DELAYED RESOLFTION AFTER LOBAR PNEUMONIA. 



Anastomosing fascicles of fusiform connective-tissue cells lie within the air vesicle and are continuous with 

 similar new formations in adjacent air spaces. 



While Micrococcus lauceolatus is the regular excitant of lobar pneu- 

 monia, other bacteria see below are not infrequently associated with 

 it, sometimes though by no means always leading to clinical complica- 

 tions and to modifications of the appearance of the lesion. Predisposi- 

 tion of the individual is an uncommonly conspicuous and important fac- 

 tor in the etiology of this as other forms of pneumonia, so that, as is well 

 known, exposure, fatigue, etc., may induce the favoring bodily condi- 

 tions under which the pneumococcus becomes harmful. The experiments 

 upon animals are most conclusive in demonstrating that intrapulnionary 

 injections of pneumococcus cultures, which may be entirely without ob- 

 vious effects in a healthy animal, may induce exudative inflammation or 

 become quickly fatal after such exposure to cold or fatigue or injury or 

 to the action of drugs, as interferes with the integrity of the blood or 

 locally damages the pulmonary tissues. l 



Delayed Resolution after Lobar Pneumonia ("Organizing Pneumonia"). 

 Instead of undergoing resolution, the fibrinous and cellular exudate in 

 the air spaces in lobar pneumonia may persist and by a process similar 

 to that which is called organization of a thrombus (page 74) may be 

 gradually replaced by new vascular fibrous tissue. Kew connective-tis- 

 sue cells grow out from the walls of the air spaces into the exudate ; these 

 cells become elongated. Intercellular fibrils develop, and long masses 

 or bands of this new tissue, often containing blood-vessels, may extend 

 for considerable distances through the air channels of the lung (Fig. 

 274). These may gradually coalesce with the walls, and together with 

 an interstitial connective-tissue growth may lead to a fibrous consolida- 

 tion of the lung. 



1 For reference to experimental excitation of pneumonia see p. 499. 



