THE MECHANICS OF THE RESPIRATORY MOVEMENTS 457 



Collapse of the Lung. A very different picture is presented 

 if the air is permitted to act upon the outside surface of the lung. 

 This end can be attained by puncturing the pleural cavity in any 

 intercostal space or by forming a communication between this cavity 

 and the respiratory passage. The former condition frequently results 

 in consequence of gunshot or stab wounds, and the latter, in consequence 

 of perforations through tuberculous lung tissue. The opening of 

 the pleural cavity is immediately followed by the retraction of the 

 external surface of the corresponding lung from the internal surface 

 of the chest wall, the intervening space being filled with air. This 

 condition which is known as pneumothorax, cannot be remedied as long 

 as the opening in the pleural cavity remains patent; in fact, the air 

 entrapped in the collapsed organ is then gradually absorbed, while the 

 formerly buoyant tissue solidifies and loses its function permanently. 

 Obviously, a lung which has lost its elasticity does not collapse 

 readily, but tends to preserve its original volume (emphysema). 



If the communication between the pleural space and the outside is 

 again closed, the air in this cavity is slowly absorbed with the result 

 that the lung gradually increases in volume until it again lies every- 

 where in contact with the chest wall and can again be subjected to 

 normal degrees of expansion. It might also be mentioned that the 

 collapse of one lung need not necessarily prove fatal, because the oppo- 

 site organ is capable of a certain degree of hyperexpansion which will 

 tend to make up for the loss sustained on the opposite side. In addi- 

 tion, it is noted that the normal organ most generally acquires a cer- 

 tain amount of new tissue which will enable it in time to perform its 

 compensatory function in a more complete manner. Attention might 

 also be called to the fact that perforating wounds of the lung are not 

 always followed by a collapse of the injured organ, because the weapon 

 may again be withdrawn without that the air has had a chance to 

 enter the intrapleural space. The diameter of the modern bullet is 

 so small and its velocity so great that the parts are scarcely lac- 

 erated and are still able to recoil and to close the defect almost 

 immediately. 



Another condition in which similar conditions prevail is pleurisy. The dry 

 stage of this inflammatory reaction having been passed, a serous exudate forms 

 upon the pleural surfaces which hi time gravitates into the most dependent portion 

 of the intrapleural space and gradually separates the visceral from the parietal 

 pleura. Eventually the lung is pushed far away from the wall of the thorax 

 until its volume scarcely exceeds that of a collapsed organ. Conditions of this 

 kind constitute hydrothorax. During the subsequent period of absorption, this 

 exudate is gradually removed with the result that the lung is slowly drawn toward 

 the chest wall until the pleural cavity is again converted into a potential capillary 

 space. In this connection mention might also be made of the fact that the com- 

 pression and resultant reduction in the respiratory capacity of this lung may be 

 relieved by withdrawing the fluid with the help of an aspirating syringe. 



Intrapleural, Intrathoracic and Intrapulmonic Pressures. The 



foregoing conditions have been discussed somewhat at length, because 



