When I was a young physician Dr. Frank Billings was one of the most distinguished figures in American medicine. I looked on him with admiration and awe. Therefore, to be asked to give this lecture, founded to perpetuate his memory, is not only an honor but also a source of great pleasure. My only regret is that I have not a more worthy tribute for the occasion. The subject of mediastinal emphysema is not a new one. Occasional cases with dramatic symptoms were reported a century or longer ago. However, during the past twenty years, especially during the past decade, experimental investigation and clinical observation have' extended our knowledge and put it on a firm base. The conditions causing mediastinal emphysema are now well known; the mechanism of its production and its relation to associated manifestations, chiefly interstitial emphysema of the lungs and pneumothorax, have been clearly exposed; the symptoms