RESPIRATION 



321 



imals the best type of recording instrument to connect with the respira- 

 tory passages is the Gad or Krogh pneumograph. All these instruments 

 must of course be calibrated, which is done by pouring a definite num- 



Fig. 109. Pneumograph. The straps (b, b) are held around the thorax, and the tube of the 

 tambour connected by rubber tubing with a recording tambour. 



ber 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. 



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

 has recently died, the air will enter 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. The degree of this negative pressure may be measured by con- 

 necting a tube and a manometer with the thoracic cavity. While the 

 thorax is at rest, as in expiration or immediately after death, this pres- 

 sure amounts to about -5 millimeters.* On inspiration it increases to -10 

 millimeters. There are therefore two problems to be considered: (1) the 

 cause of the negative pressure 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. 



