Macklin and Macklin 343 



air. The mass escape may be through a rupture in the visceral plema, but we 

 have found no evidence of this mode except under drastic experimental con- 

 ditions not comparable to clinical ones. In otir experience it is via the hmg 

 root into the mediastinum, so widening the airblock field. It is certain that 

 air bubbles are forced into the mediastinum, traveling thence along the sheaths 

 of the great pulmonary blood vessels. Unfortunately, there is a considerable 

 amount of air which cannot be so evacuated, because, as we have just seen, 

 it is trapped in regions of the connective tissue of the lung such as the inter- 

 lobular septa. If the pulmonic airblock involves one lung only, its prognosis 

 is less serious than when both lungs are implicated. No artifice has been sug- 

 gested for removing the air from the lung interstitium. 



Mediastinal Emphysema and Its Alleviation 



Mediastinal emphysema, or pneumomediastinum as it is often called, is seen 

 to be the result of invasion of air, via the lung root, from an initial PIE. The 

 tendency is for alveolar-base air leakage to go on, once it gets started, until a 

 counterbalancing condition obtains. Air-bubble columns continue to move 

 along lines of least resistance toward the lung root, and so a mediastinal cm- 

 ])hysema is likely to grow in severity. The pressure rises, and cardiac and vascu- 

 lar functions are handicapped.'' Easement may be naturally afforded to the 

 compressed contents by efflux of air through perivascular channels which it 

 dissects for itself into the root of the neck and retroperitoneum. The holding 

 strength of the mediastinal walls may be taxed to the breaking point. In rab- 

 bits a rupture of the lateral mediastinal wall, giving rise to a pneumothorax, 

 has been demonstrated.'" Instant and dramatic deliverance of the stressed medi- 

 astinal contents is effected by such a sudden breakout of air. A pneumothorax, 

 too, on the side of an increasing PIE, has the effect of stopping the leakage, 

 and hence artificial pneumothorax, at least partial, has been suggested as a 

 therapeutic measure.""'* Lateral roentgenograms are useful in revealing pre- 

 cordial air pockets, which may be evacuated by means of a hollow needle.'' 

 The anterior mediastinum is the place of election for direct attack upon the 

 cause of airblock. An understanding of the morphodynamics of airblock has 

 led to suggestions,""-* already successful,'' ' for its alleviation. It seems probable 

 that lives have already been saved by recourse to intervention. Again, func- 

 tional anatomy has aided pathology and also practical medicine. 



Hyperatmospheric Mediastinal Pressure.— Olx\\n?^x\\y the pressure in the 

 mediastinum is lower than that of the atmosphere.'" In some cases of pneu- 

 momediastinum it becomes higher. For instance, in Gumbiner and Cutler's 

 case' the plunger of the syringe shot out when they tapped the substernal 

 pocket of air, and this w\as in a newborn babe. How can we account for this 

 dramatic elevation? The mechanism seems analogous to that in the so-called 

 "pressure pneumothorax." The motive power, in nonexpcrimcntal cases, must 

 ultimately be in the individual's own respiratory musculature, regular and 

 accessory. Normally the interstitial tissue of the lung and mediastinum is pro- 



