304 



THE RESPIRATION 



her of c.c. of water from a graduate into a bottle with which the record- 

 ing instrument is connected by tubing. The displacement of the writing 

 point gives us the necessary data for standardization. 



The Intrapleural Pressure 



The air which we have just been considering depends for its move- 

 ment in and out of the air passages upon changes occurring on the outer 

 aspect of the lungs in the space between them and the thoracic wall. 

 This is called the intrapleural space. It does not really exist as an 

 actual space in the living animal, for the visceral pleura which covers 

 the lungs is in accurate and intimate apposition with the parietal pleura 

 on the inner aspect of the thorax. 



Fig. 109. Body plethysmograph for recording respiration. 



Priestley.) 



(From J. S. Haldane and J. G. 



If the thoracic walls are punctured in a living animal or in one which 

 has recently died, the air will rush into the thorax, the two layers 

 of pleura separate, and the lungs collapse, causing temporarily a space 

 to be formed between the two layers of pleura and indicating that a 

 certain subatmospheric or negative pressure must exist in the intact 

 thorax to prevent the lungs from collapsing. The degree of this nega- 

 tive pressure may be measured by connecting a tube and a manometer 

 with the thoracic cavity. While the thorax is at rest, as in expiration 

 or immediately after death, this pressure amounts to about -5 milli- 

 meters.* On inspiration it increases to -10 millimeters. There are there- 

 fore two problems to be considered: (1) the cause of the negative pres- 

 sure in the quiescent thorax, and (2) the cause of the increase of the 

 negative pressure during inspiration. 



*The minus sign indicates that the pressure is negative or subatmospheric. It is a suction pressure. 



