478 RESPIRATION 



by the elastic recoil of the lung tissue, which equals 755 mm. Hg. 

 During quiet inspiration it attains a value of 9 mm. Hg and during 

 forced inspiration a value of as much as 30 or 40 mm. Hg. 



The intrapulmonic pressure pursues a similar course. It falls 

 during inspiration and rises during expiration, but remains always 

 above the former. As the chest is expanded, the pressure in the pul- 

 monary passage falls below that of the air without, initiating a rapid 

 inflow of air which does not cease until an equalization has been ef- 

 fected. Quite similarly, the expiratory movement places the air in 

 the respiratory passage under a pressure higher than that of the at- 

 mosphere, and gives rise to an outflow of air which does not cease until 

 the pressures have been equalized. It need scarcely be mentioned that 

 the cause of these changes in the intrapulmonic pressure is to be sought 

 in the resistance encountered by the air in its passage through the 

 relatively narrow tracheal communication, and especially in its flow 



763 



< Inspiration-* /'a 

 760 m?n. ^~ 



<- Expiration . > 



7JQ 



A. INTRA-PULMONIC PRESSURE. 

 760mm 



7S1 



B. INTRA-THORACIC PRESSURE. 



FIG. 247. REPRESENTING THE CHANGES, 1, IN THE INTRAPULMONIC, AND 2, IN THE INTRA- 

 THORACIC PRESSURE DURING INSPIRATION AND EXPIRATION. 



through the glottis. Quite naturally, any condition which lessens 

 the lumen of the upper portion of the respiratory passage, must tend 

 to augment these variations in the intrapulmonic pressure. It must 

 also be evident that these changes may be intensified by breathing 

 more forcibly. Under ordinary conditions, however, the inspiratory 

 fall in intrapulmonic pressure amounts to only 1.5 mm. Hg, while its 

 expiratory rise rarely exceeds 2.5 to 3 mm. Hg. During forced respi- 

 ration much higher values are obtained. Donders, for example, was 

 able to cause a fall of 57 mm. Hg and a rise of 87 mm. Hg, the pressure 

 being registered in this case by a manometer which was connected 

 with one nostril, while the other was held shut. 



The intra-abdominal pressure is registered by inserting a hollow 

 probe among the superficial coils of intestine and connecting its free 

 end with a water manometer. During quiet respiration it retains 

 a value very close to zero, 1 but naturally, the active participation of 



1 Emerson, Archiv of Int. Medicine, vii, 1911, 1. 



