RESPIRATION 



171 



9. Mechanics of the breathing movements. A knowledge of 

 the mechanism of the breathing movements is of much practi- 

 cal importance, especially in hygiene, and may be understood 

 without great difficulty by the study of the model shown in 

 Fig. 79. The trachea and the bronchi are represented by the 

 glass tube, and the lungs by an elastic bag, Z, at the end of 

 the tube. The lungs lie in the large air-tight thorax, which in- 

 closes the pleural, or thoracic, cavity (p. 10). This thoracic wall 

 is represented in the model by a glass bell jar closed beneath 

 by a sheet of thick rubber, D. The cavity 

 of the bell jar represents the pleural 

 cavity, and the rubber represents the dia- 

 phragm (see Fig. 154). The condition of 

 the lung in the pleural cavity may be 

 still further imitated in the model by fas- 

 tening the .inflated rubber bag tightly into 

 the jar. 1 The rubber bag remains moder- 

 ately inflated within the air-tight cavity 

 of the bell jar. In the body the distended 

 lungs virtually fill those portions of the 

 thoracic cavity not occupied by the heart, 

 great blood vessels, and other organs. 2 



Now enlarge the "thoracic cavity" of the model by pulling 

 downwards on the sheet of rubber which represents the dia- 

 phragm. The " lungs " within will expand while air is sucked 

 through the glass " trachea " and mixes with that in the model 

 " lungs." When the pull is released, the " diaphragm " rises, 

 thus diminishing the size of the " thorax " and so forcing air 

 out of the " lungs." In this way the mechanism of the venti- 

 lation of the lungs may be imitated in essential particulars. 



1 Loosen the rubber stopper and, while the neck of the bell jar is open, 

 inflate the rubber bag through the tube ; while the bag is thus inflated, 

 push the rubber stopper down into the neck of the bottle. 



2 The student is again warned against supposing that the pleural cavity 

 is a large space filled with air ; in this respect the model is misleading, since 

 the lungs and other organs completely fill the thoracic cavity. 



FIG. 79. Model of the 

 action of the thoracic 

 walls and lungs in res- 

 piration (see sect. 9) 



