394 TEXT-BOOK OF PHYSIOLOGY. 



that with one nostril closed and a mercurial manometer inserted into the 

 other the pressure by voluntary efforts could be made to fall 57 mm. during 

 inspiration and to rise 87 mm. during expiration. The changes in intra- 

 pulmonic pressure are graphically represented in the upper half of Fig. 192. 

 The intra-thoracic pressure also varies during both inspiration and expira- 

 tion. As the thorax enlarges and the intra-pulmonic pressure falls, the 

 recoil of the elastic tissue increases, with the result of still further dimin- 

 ishing the intra-thoracic pressure, until its maximum is reached near 

 the end of the inspiration. The fall of intra-thoracic pressure at 

 the end of a quiet inspiration reaches to about 9 mm. Hg. In 

 forcible inspiratory efforts this fall in intra-thoracic pressure may 

 amount to 30 or 40 mm. of Hg. As the thorax again diminishes and the 

 intra-pulmonic pressure rises above the atmospheric pressure, the recoil 

 of the elastic tissue is again opposed, with the result of increasing the 



760 m?n. 



Inspirati<ut+ /^ 



z \?nm^^ . Expiration. 

 A. INTRA-PULMONIC PRESSURES. 



760mm ^-^J ^^ 7 -*- Expiration 



Xtt 



B. INTRA-THORACIC PRESSURE. 



FIG. 192. REPRESENTING THE CHANGES, i, IN THE INTRA-PULMONIC, AND 2, IN THE INTRA-THO- 

 RACIC PRESSURES DURING INSPIRATION AND EXPIRATION. 



intra-thoracic pressure, until the former condition of pressure has 

 been regained at the end of the expiration. Neither the fall nor 

 the subsequent rise of the intra-thoracic pressure takes place, however, 

 in a steadily progressive manner for the following reasons: If a tracing 

 were made of the variations in the circumference of the thorax during a 

 respiratory movement it would resemble in its main features the tracing in 

 Fig. 194, and variations in any linear dimension of the lung would be of 

 course in the same proportion. This amount of elongation of elastic tissue 

 in any direction would likewise be proportional to the force of elastic recoil. 

 Therefore the intra-thoracic pressure would vary from a uniform decrease 

 and increase just as the curve of Fig. 194 varies from uniform straight lines. 

 The changes in intra-thoracic pressure are graphically represented in 

 Fig. 192, B. 



The intra-thoracic pressure and its variations influence favorably the 

 flow of lymph through the thoracic duct (see page 222), as well as the flow 

 of blood from the extra-thoracic veins into the intra-thoracic veins, the right 

 side of the heart, and the cardio-pulmonary vessels. (See paragraphs at the 

 end of this chapter.) 



The succession of events in the thorax at the time of a respiratory act 

 may be summarized as follows: 



