BRONCHITIS. 613 



obstruction is complete from the commencement, the air is 

 absorbe d. 



It had been supposed by Laennec that the emphysema, or, 

 more correctly, the over-distension with air of the parts surround- 

 ing the collapsed lobules, was due to what he thought a 

 fact, that the act of inspiration was more powerful than 

 that of expiration, so that though air could be drawn through 

 the obstruction, it could not be breathed out. In consequence, 

 it accumulated in the ultimate pulmonary vesicles, became 

 expanded by heat, and so acted mechanically as a dilator. Dr. 

 Gairdner, however, pointed out that expiration is a much 

 more powerful act than inspiration, and that there is never any 

 difficulty in causing expulsion of air, provided always there be 

 no obstruction in the tubes. Emphysema, then, does not occur 

 in the vesicles connected with obstructed tubes, but in those 

 which are adjacent. "When the lungs are free from disease the 

 column of air presses equally in all the tubes and vesicles ; but 

 when one portion connected with any obstruction is collapsed, 

 then the adjacent parts are over expanded, so as to occupy the 

 space previously filled by the former. 



At a later , stage the contents of the obstructed bronchi are 

 pushed by the weight of the descending or inspired atmosphere 

 into the most minute bronchi, alveoli, and air vesicles, always 

 from the centre towards the periphery, and appear as minute 

 white points beneath the pleural surface. They are well shown 

 in the figure. 



Fig. 38. — Pleural aspect of pulmonary lobe from American ox slaughtered 

 at Liverpool ; alveoli filled with muco-purulent matter ; pleural surface intact. 

 The microscopic examination revealed broncho-pneumonia in some of the alveoli 

 (see fig. 40) ; whilst others showed no traces of inflammation (see fig. 36), but 

 were merely filled with the inhaled bronchial secretions. 



