136 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



of the chest. Observe that this space is widened on inspiration and 

 that the lower borders of the lungs are then drawn into it. Trans- 

 illuminate this region, so that individual alveoli may be made out. Note 

 that the tissue of the lung is in absolute contact with the tendinous 

 portion of the diaphragm. 



5. Intrapleural Pressure. Procure a water manometer. Color the 

 water with a little indigo-carmin. Connect the central tube of this 

 manometer with a metal cannula, about 10 cm. in length and curved 

 at its end. Force the end of this cannula through the soft tissues of 

 the seventh intercostal space and immediately turn it so that it comes 

 to lie flat between the surface of the lung and the chest wall. Do not 

 move it excessively, because this might give rise to a leakage of air 

 into the intrapleural space and a collapse of the lung. 



Observe that the liquid in the manometer is drawn toward the 

 chest as soon as the end of the cannula has perforated the wall of the 

 thorax and has forced the lung tissue away from the inner surface of the 

 chest wall. The end of the cannula thus comes to lie between the 

 visceral and parietal layers of -the pleura. The air in this artificial 

 cavity, and hence also the liquid in the manometer, is exposed to the 

 elastic recoil of the lung tissue. The force of this recoil is indicated by 

 the inward movement of the liquid (cm. H^O). Inasmuch as the lung 

 tissue is more highly stretched during the inspiratory period, this press- 

 ure must fluctuate. It approaches zero (line of atmospheric pressure, 

 760 mm. Hg) at the end of expiration and assumes a value of as much 

 as 5 mm. Hg (756 to 755 mm. Hg) on quiet inspiration. 



Partially occlude the rubber tube attached to the tracheal cannula. 

 As the breathing assumes a labored character, these differences in the 

 intrapleural pressure become more apparent ( 8 to 10 mm. H, in 

 cats), because the lung tissue is now put under a greater elastic tension 

 than during normal respiration. 



6. Collapse of the Lung. Inspect the tendinous portion of the dia- 

 phragm through the wound in the abdominal wall. Note that the pink 

 pulmonary tissue lies in absolute contact with it. Twist the cannula 

 slightly, allowing air to enter the pleural sac. The lung tissue will then 

 be seen to recoil from the diaphragm (pneumothorax) . This procedure 

 most generally leads to the collapse of only one lung, while the other 

 lung remains expansible and is capable of effecting an adequate inter- 

 change of the gases. Discuss the effect of placing the chest and posterior 

 part of the animal in a compartment in which a negative pressure may 

 be produced equaling the intrapleural. 



Open the cavity of the thorax by a cut through the median line of 

 the sternum. Institute artificial respiration through the ether bottle. 

 Note the position of the cannula in the now actual and greatly enlarged 

 intrapleural space. Withdraw it. 



Temporarily discontinue the artificial respiration. Observe that the 

 respiratory muscles continue to contract in spite of the fact that the 

 lungs can no longer be expanded. As the CO2 accumulates in the 



