612 SPORADIC DISEASES. 



congestion and subsequent inflammation ; consequently it is 

 found that broncho-pneumonia often succeeds bronchitis, due 

 to the absence of the expansion and contraction of the air 

 vesicles which normally aid the pulmonary circuLation, and to 

 arrestment of the blood-flow owing to imperfect a3ration. This 

 congestion is soon succeeded by effusion of serum, and the 

 bluish-purple collapsed portions become darker in colour and 

 less resistant in consistence. They, however, retain some degree 

 of elasticity, for, if not too rudely pulled out, they do not tear 

 as in pleuro-pneumonia ; if cut into and exposed to the atmo- 

 sphere for a few minutes, the bluish-purple colour becomes bright 

 scarlet. It is important to bear in mind that the pneumonic 

 process which supervenes in bronchitis is principally confined to 

 those portions of the lungs in which collapse has taken place. 

 Sometimes the collapse is isolated, invading but small portions 

 of the lungs : this condition is not rarely witnessed in parasitic 

 bronchial disease. These limited collapsed portions vary in 

 size, are rather wedge-shaped, and have their apices towards 

 the obstructed bronchus. The lung tissue surrounding them 

 may be more or less congested, or it may be emphysematous, 

 but no juice is exuded from them when cut into, as in acute 

 pleuro-pneumonia. 



Professor Gairdner was, I believe, the first to show that con- 

 densation of the vesicular substance occurs as a result of mucous 

 or other obstruction in the air-tubes leading to the condensed 

 portion. It is at first sight difficult to understand how incom- 

 plete obstructions of the bronchi — and these obtain much more 

 frequently than absolutely complete occlusion — cause collapse. 

 One would suppose that some quantity of air would gain access 

 into the vesicles, but such is apparently not the case ; and it 

 seems that the air gradually finds its way out by the edges of 

 the obstructing substance. The expiratory force, so long as there 

 is air in the vesicles, constantly tends to dislodge the obstructing 

 body by pushing it towards the wider (proximal) end of the tube, 

 whilst the inspiratory drives it inwards towards the narrower 

 tubes, which it effectually occludes. The entrance of air is thus 

 more or less eff'ectually opposed, and its exit permitted, so that 

 ultimately the vesicles beyond become completely emptied ; in 

 fact the plug acts as a valve, allowing the air to pass in one 

 direction, but opposing its passage in the other. Where the 



