334 MANUAL OF PHYSIOLOGY. 



the abdomen, which, associated with the intercostals and quadratus 

 lumborum, are the most powerful agents in drawing down the 

 thoracic walls. 



FUNCTION OF THE PLEURA. 



From what has been already said it is obvious that by far the 

 greatest amount of movement takes place in the lower part of the 

 thorax, while the capacity of the apex changes but little. The 

 space formed in the chest during inspiration is practically formed 

 between the costal wall and the diaphragm (compare Figs. 148, 149). 

 If the lungs and the walls of the thorax were fused together, with- 

 out the interposition of serous membranes, the different parts of 

 the lungs would have to follow the movements of that part of the 

 thorax to which they are attached. Thus the lower parts of the 

 lung would be much distended during inspiration, and the apices 

 would receive but little addition to their contained air. This con- 

 dition is often found in disease of the pleura, leading to adhesion 

 of the visceral and parietal layers. When such cases live for some 

 time after the pleurisy and the adhesions persist, the air-cells of 

 the lower margins of the lungs are commonly found to be dis- 

 tended and bloodless (i.e., local emphysema from habitual over- 

 distension) ; while on the other hand, the apices become abnormally 

 dense, and the alveoli are contracted and airless. 



The surface of the soft elastic lung tissue is normally quite free, 

 being encased in a serous membrane, the smooth surface of which 

 can slide uninterruptedly and freely over the similar lining of the 

 costal wall. That this motion of the lung actually occurs may be 

 seen from watching the lung through the exposed parietal pleura, 

 or recognized by studying the sounds produced by a roughness of 

 the pleura, such as occurs in inflammation, when a "friction" can 

 be detected by the ear. 



The lungs move in a definite direction. From the most fixed 

 points of the thorax, namely, the apex and vertebral margin, they 

 pass towards the more movable inferior costal and sternal regions. 

 In short, the anterior part of the lungs passes downwards and for- 

 wards to fill up the gap made by the descent of the diaphragm 

 and by the passing of the costal wall upwards and forwards. 



