BEONCHITIS. 311 



BRONCHITIS AND BRONCHO-PNEUMONIA. 



The mucous membrane of the bronchi being continuous with 

 that of the nasopharynx and fauces, bronchitis may arise by 

 extension of infectious processes from above. Air-borne 

 irritants have direct access to the air tubes, and in certain 

 conditions particles of food from the pharynx may be aspirated 

 into the respiratory passages. The air tubes are supplied with 

 blood by the bronchial arteries, whilst the pulmonary arteries 

 distribute blood to the terminal bronchioles and the alveoli. The 

 bronchial arteries differ from the pulmonary in that they are 

 subject to vaso-motor control, consequently vascular changes 

 may be induced in the bronchi by a variety of reflex causes. 

 Lastly, the bronchial arteries, as part of the general vascular 

 system, may themselves carry infection to the bronchi, and this 

 explains the occurrence of generalised bronchitis in certain of the 

 infectious diseases. 



An impervious basement membrane lies beneath the epithelial 

 layers of the bronchi, and this membrane probably plays an 

 important part in the course of bronchitis. Leucocytes and 

 foreign particles cannot penetrate it, as it is not traversed by 

 lymphatics; consequently such particles and the secretions of 

 bronchitis are not absorbed but are passed upwards by the 

 ciliated stream and expectorated. 



The walls of the lobular bronchioles are thin and their lumina 

 are small. Inflammatory affections of these parts tend to assume 

 a suffocative character and to spread to the air cells, giving rise 

 to broncho -pneumonia. Although there is no lymphatic absorp- 

 tion from the bronchi, absorption can take place from the alveoli 

 and air cells. 



The bronchial veins play practically no part in returning blood 

 from the lung, as at the most they only drain the tissues of the 

 root. All the blood from the lung is returned by the pulmonary 

 veins, and consequently is poured into the left auricle of the heart. 

 Hence there is an anatomical reason for the early occurrence of 

 passive pulmonary congestion in association with diseases of the 



