THE PLEURA 237 



closely applied to the lung, forming the pulmonary pleura; the other 

 side fits as closely to the ribs, forming the costal pleura. Within 

 the sac is a small quantity of serous fluid (secreted by the endothe- 

 lium of the pleura), which prevents friction when the ribs move 

 and the lungs expand or contract. 



Although the bony thorax is bounded above by the first rib, 

 the thoracic cavity extends an inch above the rib on each side, 

 bounded by an expansion of the costal pleura and lodging the apex 

 of the lung. 



R. mammary 

 artery 



L. phrenic nerve 



m ma ^Mom^BHH I^MI <m 



R. phrenic nerve 



FIG. 161. THE PLEURAL SACS. 



The dotted lines indicate the pleural sacs, with space between the layers. 



(H olden.) 



Clinical note. If the pleura becomes inflamed the quantity of fluid 

 diminishes and the surfaces rub together, causing acute pain and a fine 

 crackling sound as of friction. This condition is pleuritis or pleurisy. 



Resume. In respiration, or the act of breathing, the inspired 

 air enters the nasal chambers, passes through the naso-pharynx, 

 oro-pharynx, larynx, trachea, bronchi, bronchial tubes, and bron- 

 . chicles, to the alveoli and air-cells or air vesicles. 



THE PHYSIOLOGY OF THE RESPIRATORY PROCESS 



The function of the respiratory apparatus is first, to accom- 

 plish an interchange in the lungs between the oxygen of the air 

 and carbon dioxide of the blood, in other words to bring nutri- 

 ment to the blood and to remove waste from it. 



