134 



ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



Set the spirometer at zero. After several normal respirations, in- 

 spire as deeply as possible and immediately expire as much as possible 

 through the mouthpiece of the spirometer. Ascertain the vital capacity. 



2. Schema Illustrating the Action of the Thorax. Examine the con- 

 struction of the model represented in Fig. 87. 



(a) Normal Respiration. Lower and raise the rubber membrane 

 closing the bell-jar. Observe that the rubber pouch representing the 

 lung is expanded during the first movement, and decreased in size 

 during the second. Clearly, these changes are brought about by an 

 action upon the entire external surface of the rubber pouch. When the 

 pressure in the "intrapleural space" (between the surface of the rubber 

 pouch and the wall of the bell-jar) is decreased by the downward move- 



FIG. 87. APPARATUS ILLUSTRATING THE EXPANSION OF THE LUNG. (Laulanie.) 



n, Bell jar; B, lung in form of rubber balloon; M, manometer in connection with 

 "intrapleural space." Another manometer may be connected with the inlet-tube to 

 register the "intrapulmonic pressure." 



ment of the "diaphragm" (rubber membrane), the walls of the "lung" 

 (rubber pouch) are pulled uniformly outward. An area of low pressure 

 is thereby established within the "lungs," forcing a certain quantity of 

 air to flow through the "trachea" (inlet tube) into the pouch. The 

 reverse relationships are established by the upward movement of the 

 "diaphragm," simulating expiration. 



The two manometers fastened to the top of this model are connected, 

 on the one hand, with the "intrapleural space" and, on the other, with 

 the "trachea." Carefully observe the variations in the pressures dur- 

 ing inspiration and expiration, showing that the fall in "intrapleural 

 pressure" is responsible for the expansion of the "lung" and the subse- 

 quent influx of air into its spaces. Normally, of course, the intrapleural 



