88 RESPIRATORY SYSTEM. - 
obstruction is removed soon, the air spaces become inflated and normal, 
But if the collapse of the walls of the air spaces persists, then inflam- 
matory changes of the exudative type are set up. New tissue may 
be formed and the affected part become solid. 
How are the inflammations of the lung classified ? 
Acute lobar pneumonia. 
Broncho-pneumonia. 
Secondary and complicating pneumonia. 
Pneumonia of heart disease. 
Interstitial pneumonia. 
Tubercular pneumonia. 
Syphilitic pneumonia. 
Describe the changes occurring in acute lobar pneumonia. 
There is a growth of pathogenic bacteria, to be described later. The 
inflammation is purely of the exudative type. The course of the dis- 
ease is divided into three stages. First, the stage of congestion, in 
which the microscopical changes are slight. Second, the stage of red 
hepatization, in which the air spaces and bronchi are filled with the 
products of the exudative inflammation, viz.: Fibrin, serum, pus, red- 
blood cells and a few epithelial cells. Third, the stage of gray hepati- 
zation, in which the red-blood cells lose their color, the various 
exudation products begin to degenerate, soften, and break down. A 
portion of this degenerated material is expectorated, the rest is 
absorbed and carried away by the lymphatics and bloodvessels. In 
this way the air spaces are cleared up, but in some of them occasionally 
there is a formation of new connective tissue which seems to pe 
permanent. 
What is the character of the lesion of broncho-pneumonia ? 
The change begins in the walls of the bronchi. They are infiltrated 
with new cells, this infiltration extending to adjacent air spaces and 
not to those in which the bronchi terminate. The affected air spaces 
become filled with pus and epithelium. In the gross specimen there 
are seen patches of solidification about the affected bronchi, but between 
these solidified portions the air spaces may still be aérated. The 
change may proceed until the affected portion of the lung becomes 
entirely solid by the extension of the inflammatory process and by the 
union of the masses of svlidification. The bronchi may be filled with 
pus, but the first change is the thickening of the walls of the bronchi. 
Occasionally the infiltration of the walls of the air spaces and bronchi 
