202 Veterinary Medicine. 



Causes. These are the same as those of catarrh and sore throat. 

 It is but the continuation of the same mucous membrane which 

 is affected in all alike, and the same atmospheric changes, hot 

 stables, noxious inhalations and exposures to cold and wet, will in- 

 duce this disease rather than the others, when the bronchial mucous 

 membrane is more predisposed. Bronchitis often supervenes 

 upon sore throat, by the extension of the inflammation down- 

 ward into the chest. 



As in the other affections of the respiratory tract, microbes bear 

 an important r&le in the causation and maintenance of the in- 

 flammation. These may be specific pathogenic organisms which 

 determine the disease whenever they are implanted in a suscep- 

 tible subject, but most commonly they are mere saprophytes liv- 

 ing habitually, in health, on the surface of the respiratory mu- 

 cosa, and only proving injurious when that membrane has been 

 injured or debilitated, or when the general tone of the system 

 . has been lowered by other conditions. As the air passages are 

 always widely dilated, in contrast with other mucosae which are 

 closed by sphincters, and as the germ-laden outer air is inhaled 

 with every breath, the floating organisms are of necessity car- 

 ried in, to attach themselves to the moist surface of the canal. 

 Though this is counteracted by the tendency of the inhaled mi- 

 crobes to strike against and adhere to the moist mucosa in nose, 

 pharynx and larynx, yet a few constantly escape and reach the 

 tubules within the lungs. These tend to reach the bronchia by 

 gravitation. When they have reached the bronchia they are 

 still opposed by the ever active ciliated epithelium which tend to 

 sweep them upward and outward, until the cilia have become 

 paralyzed and inactive through the supervention of inflamma- 

 tion. There is the further antagonism of the epithelial cells 

 themselves which exercise their selective powers in excluding 

 the microbes from the mucosa, and in destroying their vitality if 

 they should succeed in making a partial entrance. This last in- 

 fluence is particularly strong in the trachea, bronchi and larger 

 bronchia where the epithelial coating is very thick and abundant, 

 and the resistance lessens by degrees as we approach to the 

 smaller bronchia and the terminal or capillary tubes in which 

 the epitheUal layer becomes very thin and permeable. A suc- 

 cessful invasion by the microbes is favored by the inhalation of 



