RESPIRATION 305 



The Permanent Negative Pressure. Let us start with the changes 

 that occur in the thorax when the first breath is drawn. While the an- 

 imal is still in utero, the lungs completely fill the thorax. When the 

 first breath is drawn the thoracic cage expands more quickly than the 

 lungs, so that the latter become stretched, the stretching force being 

 the air that is introduced into them from the outside through the tra- 

 chea and bronchial tubes. On becoming stretched the lungs fill the 

 increased space created in the thorax by the greater expansion of the 

 thoracic cage. This in itself, however, would not explain the cause of a 

 subatmospheric pressure in the intrapleural space. Another factor must 

 come into play namely, the elastic tissue of the lungs, which by the 

 expansion will become stretched and, therefore, tend constantly to re- 

 lax to its previous condition and so exert a pull on the structures be- 

 tween it and the thoracic wall. It is this elastic recoil which we really 

 measure when we connect a manometer with the intrapleural space. 

 Throughout life the lungs remain of smaller size than the thoracic wall, 

 and therefore to fill the thoracic cavity they are constantly more or 

 less distended and the elastic tissue somewhat stretched. The lungs 

 are, however, not the only structures in the thorax which become ex- 

 panded; all thin -walled vessels and viscera, like the veins, the esopha- 

 gus, the auricles, etc., must also become opened out a little. 



When the thoracic wall is punctured and the outside air allowed free 

 entry to the intrapleural space, differences in pressure no longer exist 

 on the inner and outer aspects of the lungs, so that they collapse into 

 the postmortem condition on account of the elastic recoil. If a puncture 

 in the thoracic wall of a living animal is immediately occluded, the 

 lungs will expand again, because the blood absorbs the gases from the 

 intrapleural space and recreates the partial vacuum required to expand 

 the lungs. This absorption of gas in the pleural cavity is usually quite 

 rapid; but if the pneumothorax, as the condition is called, is allowed to 

 persist for any length of time, the lungs will not become properly ex- 

 panded again. 



The Greater Negative Pressure on Inspiration. The cavity of the tho- 

 rax becomes increased in all diameters during inspiration, with the re- 

 sult that a greater space in the pleural cavity has to be filled. All the 

 thin-walled structures in the thorax therefore become still more stretched, 

 the lungs of course participating to the greatest extent because of the 

 entrance of outside air. The stretching of the elastic structures causes 

 a greater pull, or negative pressure, to be exerted in the pleural cavity. 

 Instead of being -5 mm. Hg, as in expiration, the intrathoracic pressure 

 now conies to be above -10 mm. Hg. 



When any obstruction exists in the air passages, the changes in intra- 



