RESPIRATION. 



359 



This negative pressure is called the intra-thoracic or intra-pleural, 

 not intra-pulmonic, pressure, and is always less than the air-pres- 

 sure. Intra-thoracic pressure is the pressure in the pleural cavity 

 and mediastinum. The pressure necessary to counterbalance the 

 elasticity of the lungs when they are quiescent in the pause of res- 

 piration is, in man, 7 millimeters of mercury, and when the lungs 

 are fully distended it rises to 30 millimeters of mercury. 



The pressure in the pleural cavity is less than an atmosphere; 

 it is a negative pressure, and is due to the fact that the lungs are 

 smaller than the thoracic cavity in which they lie. 



4' 5 



Fig. 127. Apparatus to Illustrate Relations of Intra-thoracic and 

 External Pressures. (After BEAUNIS.) (From Mills's "Animal Physi- 

 ology," copyright, 1889, by D. Appleton and Company.) 



A glass bell-jar is provided with a light stopper, through which passes a 

 branching glass tube fitted with a pair of elastic bags representing lungs. The 

 bottom of the jar is closed by rubber membrane representing diaphragm. A 

 mercury manometer indicates the difference in pressure within and without the 

 bell-jar. In the left-hand figure it will be seen that these pressures are equal; 

 In right (inspiration), the external pressure is considerably greater. At one 

 part (6) an elastic membrane fills a hole in jar, representing an intercostal 

 space. 



It follows, then, that with a full inspiration the pressure exerted 

 upon the cardiac organs in the chest is 30 millimeters less than that 

 of the air-pressure of 760 millimeters of mercury. 



When the waves of blood-pressure are compared with the curves 

 of the movements of respiration or with the variations of intra- 

 thoracic pressure, it is found that, while arterial tension rises dur- 

 ing inspiration and falls during expiration, neither the rise nor the 

 fall is exactly synchronous with either inspiration or expiration. 



In inspiration, the flow of blood from the veins outside the chest 

 is pressed towards the inside of the chest, because the air-pressure 



