MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 237 



elastic fibres are still stretched; and the tension of these acts in the 

 opposite direction to the external atmospheric pressure, and dimin 

 ishes by its amount the pressure inside the thoracic cavity. In th^ 

 dead body Bonders measured the value of this tension, and there- 

 fore of the negative pressure of the thorax, by tying a manometer 

 into the trachea, and then causing the lungs to collapse by opening 

 the chest. It varied from 7-5 mm. of mercury in the expiratory 

 position to 9 mm. in the inspiratory. So far as can be judged from 

 observations made on persons suffering from various diseases of the 

 respiratory organs, the alterations during ordinary breathing do not 



Fig. 116. Variations of Intraihoracic Pressure. Upper curve>*carotid blood-pres- 

 sure (dog) lower curve, intrapleural pressure. At 42 the trachea was closed ; 

 the blood-pressure curve shows the rise of asphyxia, and the intrapleural curve, 

 greatly exaggerated pressure variations due to the strong and slow but abortive 

 respirations. 



amount to more than 3 or 4 mm. of mercury. But when an attempt 

 is made in the dead body to imitate a deep inspiration by making 

 traction on the chest walls so as to expand the lungs, the intra- 

 thoracic pressure may fall to 30 mm. of mercury; and in a living 

 rabbit, during a deep natural inspiration, a pressurs of 20 mm. 

 has been seen. 



The reason why the lungs collapse when the chest is opened is 

 that the pressure is now equal on the pleural and alveolar surfaces, 

 being in both cases that of the atmosphere. There is therefore 

 nothing to oppose the elasticity of the lungs, which tends to con- 

 tract them. So long as the chest is unopened, the pressure on the 

 pleural surface of the lunge is less than that on the alveolar surface 



