PASSIVE HYPER^EMIA. 337 



of the air-spaces may be so great as to be apparent to the 

 naked eye. 



Microscopically the air-vesicles are found to be rounded in- 

 stead of polygonal, and are increased in diameter. The 



FIG. 147. 



Section from an emphysematous lung (Ribbert). The pulmonary alveoli are en- 

 larged; their walls are stretched and thinned; atrophied because of repeated 

 excessive air-pressure within the alveoli. In more extreme cases of emphysema 

 the atrophy of the alveolar walls may lead to their total destruction in places, 

 so that the cavities of neighboring alveoli communicate. (Compare with Fig. 

 149.) 



alveolar walls are thinned (see Fig. 147) ; and the bloodvessels, 

 being stretched over a larger surface, are narrowed, the 

 anaemia of the lung-tissue thus produced further favoring the 

 atrophy and degenerative changes in the alveolar septa. As 

 a remote consequence of the increased pulmonary blood -pressure 

 there is hypertrophy of the right heart. 



PASSIVE HYPER^SMIA. 



Passive hypersemia, brovm induration, or brown oedema, re- 

 sults from some obstruction to the outflow of blood from the 

 lungs, most frequently from some myocardial degeneration, or 

 valvular lesion, especially mitral; though it may occur in 

 connection with aortic stenosis, or regurgitation. 



22-Hist. 



