PRESSURE CONDITIONS IN LUNGS AND THORAX. 655 



child has practically no reserve supply of air in the lungs; at each 

 expiration the lungs are entirely emptied (except for the minimal 

 air). The ventilation of the lung alveoh is correspondingly more 

 perfect than in older persons. With the development of a negative 

 pressure it would seem that the expanded lung would be pro- 

 tected from any danger of a return to the fetal state of consohda- 

 tion. The atria and alveolar sacs must always be dilated with 

 air even at the end of a forced expiration, so long as the bronchioles 

 leading to them remain patent. If, however, for any reason the 

 alveolar ducts or respiratory bronchioles were blocked for any 

 length of time, by mucus, for example, or a contraction of the 

 muscular ring, the air captured in the atria and air-sacs might be 

 absorbed partly or completely, giving a condition of local con- 

 solidation or incomplete expansion known as atelectasis. 



Pneumothorax. — When the pleural cavity on either side is opened 

 by any means air enters and causes a greater or less shrinkage of 

 the corresponding lung. This condition of air within the pleural 

 cavity is designated as pneumothorax. It is evident that air may 

 enter the pleural cavity in one of two general ways: By a puncture 

 of the parietal pleura such as may be made by gunshot or stab 

 wounds in the chest, or by a puncture of the visceral pleura, such as 

 may occur, for example, by the rupture of a tubercle in pulmonary 

 tuberculosis, the air in this case entering from the alveoli of the 

 lungs. From the physical conditions involved it is evident 

 that if the opening into the pleural cavity is kept patent then the 

 lung will collapse completely and eventually will become entirely 

 solid, since the small amount of entrapped minimal air will be 

 absorbed by the blood. The other lung, the heart, etc., will also 

 be displaced somewhat from their normal position by the unusual 

 pressure. If, however, the opening is closed, then the air in the 

 pleural cavity may be absorbed completely by the circulating blood 

 and the lung again expand as this absorption takes place. In 

 human beings pneumothorax occurs most frequently in conditions 

 of disease, particularly pulmonary tuberculosis, and the air in the 

 thorax is associated also with a liquid effusion, this combination 

 being designated somethnes as hydropneumothorax.* 



The Aspiratory Action of the Thorax. — The negative pres- 

 sure prevailing in the thoracic cavity must affect the organs in the 

 mediastinal space. The intrathoracic portion of the esophagus, 

 for instance, is exposed, at times of swallowing at least, to a full 

 atmosphere of pressure on its interior, while on its exterior it is under 

 the diminished intrathoracic pressure. This difference tends to 

 dilate the tube and may aid in the act of swallowing. The main 



* See Emerson, "Pneumothorax," Johns Hopkins Hospital Reports. 

 11, 1, 1903. 



