When reduced to fundamental mechanics, the act of inspiration is a simple one which is accomplished by a sudden enlargement of the thorax and a consequent diminution of pressure within it. Air, therefore, rushes down the trachea so that the pressures on the outside and the inside of the thorax will be more nearly equalized. Similarly, the act of expiration is the reverse of that of inspiration. By muscular action the thorax is diminished in size, the intrathoracic pressure is increased, and air is forced out of the trachea. Changes of intrathoracic pressure, therefore, loom large as factors necessary in respiration and consequently in the maintenance of life.
It is a curious paradox that although the recognition of the importance of maintaining the normal intrathoracic pressure relationships was an effective barrier against the development of surgical procedures in conditions of the chest until recently, now procedures which result in at