INFLAMMATION 



307 



border of the pulmonary pleura, recede from the anterior fold of the 

 parietal pleura, and glide backwards upon its surfaces, which fall together 

 at the same instant, until a new inspiratory effort separates them (fig. 

 98). In just the same way, every other point of the surface of the lung 

 is shifted forwards and downwards to a definite extent during inspira- 

 tion, backwards and upwards to a corresponding extent during expira- 

 tion; the degree of its displacement being regulated by its distance 

 from the motionless apex on the one hand, and the equally motionless 



Fig. 97. 



Diagrammatic section through the left half of the thorax, at the 

 end of a deep inspiration. L. Lung ; B. Chest-wall ; h. 

 Heart ; w. Vertebra? ; m. Mediastinum. 



posterior border'on the other. Supposing any given point of the pulmo- 

 nary pleura {a) to be wetted by the irritant fluid, it is obvious that this 

 point will advance during inspiration and will contaminate a hitherto 

 unaffected portion of the>pposed surface of the parietal pleura (a— ^). 

 During the expiration which follows, a portion of the pulmonary pleura 

 {a, I) which lies in front of the point originally involved, becomes iri- 



