RESPIRATION. 393 



eleventh rib, inserting itself into the space developed between the thorax 

 and diaphragm as the latter contracts and is drawn away from the former. 

 In consequence of the lateral expansion the anterior border of each lung 

 advances toward the middle line until the heart is almost covered. With 

 the beginning and continuance of expiration the lungs exhibit a reverse 

 movement which continues until they reach their original position. At all 

 times, however, the movements of the lungs are entirely passive and deter- 

 mined by the movements of the thorax. 



The Changes in the Relation of the Thoracic Organs, and in the In- 

 tra-pulmonic and Intra-thoracic Pressures. In the dynamic condition, as 

 previously stated, the relations of the thoracic organs undergo a change as 

 well as the intra-pulmonic and intra-thoracic pressures. Thus during 

 inspiration the diaphragm descends, the ribs ascend and outwardly rotate 

 and the sternum advances, the result of which is an enlargement in the 

 diameters of the thorax. Coincidently with the enlargement of the thorax 

 through muscle activity there goes a corresponding increase in the size and 

 capacity of the lungs, in consequence of the expansion and pressure of the 

 air in the pulmonary alveoli. 



During expiration the diaphragm ascends in consequence of the return 

 of the displaced abdominal viscera, the ribs descend and inwardly rotate 

 and the sternum recedes from the recoil of the elastic tissues, the result of 

 which is a diminution in the diameters of the thorax. Coincidently with the 

 diminution of the thorax there goes a decrease in the size and capacity of the 

 lungs in consequence of the recoil of their elastic tissue whereby the air in 

 the lungs is compressed. 



The intra-pulmonic pressure in consequence of the alternate expansion 

 and compression of the intra-pulmonic air also undergoes a considerable 

 variation. 



During inspiration the intra-pulmonic air expands. With the expansion 

 its pressure falls; but though it is now less than atmospheric pressure it is 

 yet much greater than the opposing force of the lung tissue. As a result of 

 the fall of intra-pulmonic pressure, there is a rapid inflow of air which con- 

 tinues until atmospheric pressure is restored; that is, at the end of the 

 inspiration. 



During expiration the intra-pulmonic air becomes compressed. With 

 the compression its pressure rises above that of the atmosphere and in conse- 

 quence there is a rapid outflow of air, which continues until atmospheric 

 pressure is again restored; that is, at the end of the expiration. (Fig. 192, A .) 



The cause for the fall of intra-pulmonic pressure during inspiration and 

 the rise during expiration is to be found in the resistance offered by the air- 

 passages to the movement of the air, throughout their entire extent, and 

 especially at the level of the vocal bands. The greater the resistance, from 

 whatever cause, physiologic or pathologic, the greater the variations of 

 the pressure. If the inspiratory and expiratory movements take place slowly 

 the intra-pulmonic pressure may scarcely vary in either direction. 



In quiet inspiration the fall of pressure, as indicated by a manometer 

 inserted into one nostril, seldom amounts to more than 1.5 mm. of Hg., the 

 rise in expiration, 2.5 to 3 mm. of Hg. In forcible inspiratory and expiratory 

 efforts these limits may be largely exceeded. Thus it was found by Donders 



