DISEASES OF THE LUNGS, 155 



Pneumonia is especially prevalent during spring and 

 autumn, when sudden changes in the atmospheric conditions 

 are of frequent occurrence. Congestion of the lungs, and 

 also previous inflammation of these organs, are both pre- 

 disposing causes of pneumonia. 



Pneumonia may also be induced by irritating gases, by 

 the smoke of burning straw or wood, or other materials, by 

 medicines, especially such irritants as ammonia, and by 

 any foreign bodies which find their way into the lungs. 

 It may result from injury to the lung through wounds in 

 the chest-walls, or by fractured ribs penetrating into the 

 lung-tissue; but pneumonia from these causes is not of 

 such frequent occurrence as might be anticipated. 



Pneumonia is met with in some specific fevers, as influenza, 

 purpura, anthrax, and some others. 



In pyaemia and glanders it is sometimes developed as 

 the result of infecting material carried to the lungs. 



Lastly, catarrh of the ordinary type, when neglected or 

 treated carelessly, often terminates in secondary pneumonia 

 by spreading of the inflammation downwards into the pul- 

 monary vesicles. 



PATHOLOGICAL CHANGES IN PNEUMONIA.— In- 

 flammation of the lungs generally commences in the in- 

 ferior portions, and invades the tissues from below upwards 

 (Williams), and in all cases the small bronchial tubes are 

 more or less involved in the process. 



The pneumonia process not unfrequently involves both 

 lungs, especially when associated with or preceded by 

 bronchial inflammation, but far more often it aff'ects one 

 side only, the right side being oftener involved than the 

 left. 



1. PATHOLOGICAL CHANGES IN ACUTE PNEU- 

 MONIA. — The local changes in acute pneumonia consist in 

 intense inflammatory hypersemia of the lungs, and in the 



