346 



PHYSIOLOGY. 



it is evident that there must be a reverse movement of the thoracic 

 walls whereby the chest-capacity is diminished. This act increases 

 the pressure exerted by the contained air, with the result that as 

 much of it is expelled along the usual avenues for its passage as i& 

 necessary to equalize the pressure upon the inside and outside of 



Fig. 117. The Action of the Ribs in Man in Inspiration. (BECLARD.) 



The shaded parts represent the positions of the ribs in repose. The line 

 A-B represents a horizontal plane passing through the sternal extremity of 

 the seventh rib; the line C-D represents a horizontal plane touching the 

 superior extremity of the sternum; the line H-G indicates the linear direction 

 of the sternum. When the ribs are elevated as indicated by the dotted lines, 

 the line A-B becomes the plane a-b, the line C-D, the line c-d, and the line 

 H-G becomes the line Ji-g, the projection of the sternum being more marked 

 interiorly. The distance which separates the line M-N from the line m-n 

 measures the increase in the antero-posterior diameter of the thorax. 



the chest. This outgoing of air constitutes the second act of respira- 

 tion: expiration. The regular succession of these two alternating 

 currents of air constitutes breathing, or respiration. 



Inspiration. Inspiration has for its motive agents the dia- 

 phragm, the intercartilagenous part of internal intercostals, the long 



