DISEASES OF EESPIKATORY ORGANS 163 



Etiology. — Many predisposing factors enter into the causation of 

 tliis disease, such as undue exposure or inhalation of irritating gases, 

 but it is probable that the specitic active cause is microbian. 



Lesions. — Croupous pneumonia is characterized by the regularity 

 of the successive changes that occur in the affected portion of the 

 lung. The disease usually involves the dependent part of the lung 

 and is essentially lobar, but more or less than this amount of lung 

 may be involved. The various stages that occur in the lung are as 

 follows: congestion, red hepatization, gray hepatization, and resolu- 

 tion. These stages occur in the order mentioned, and there is no 

 distinct line of demarcation between each succeeding stage. The 

 length of duration of each stage is subject to variation, but on the 

 whole is quite constant. The congestive stage is of about twenty-four 

 hours' duration. During this stage the affected portion of the lung 

 is engorged with blood. In the beginning the air cells contain air 

 but as the disease progresses the alveolar capillaries begin to leak 

 or there is a hemorrhagic exudate into the alveoli. Red hepatization 

 begins when exudation is evident, and this stage persists for from 

 three to five days; the area affected is red, dense, heavy, and liver- 

 like. Gray hepatization is characterized by a gray or yellow colora- 

 tion of the affected lung. This stage succeeds red hepatization and 

 is due in part to the substitution of leukocytes for the coagulated 

 hemorrhagic exudate that characterizes red hepatization and in part 

 to chemical change of the hemoglobin of the extravasated red blood 

 cells in the hemorrhagic exudate. The lung tissue affected with gray 

 hepatization is gray in color, dense, heavy, and cuts like liver tissue. 

 Gray hepatization persists for from three to five days, and when the 

 alveoli are relatively free of exudate and leukocytes the lung is said to 

 be in a stage of resolution, which includes that period, of time during 

 which the air cells are freed of all inflammatory products and return 

 to the normal condition. This is a variable period and depends upon 

 the resistance of the affected animals. The lung, during this stage, 

 contains some inflammatory products in the beginning and later 

 small quantities of mucus and debris, but in the final stages the 

 lung is practically normal. 



Symptoms. — Inappetence, high temperature, and increased respira- 

 tion and heart action characterize croupous pneumonia. On the 

 second or third day a ' ' prune-juice ' ' nasal discharge may be observed 



