144 RESPIRATION 



act, but it is probable that they serve only to maintain the 

 proper degree of tension of the intercostal tissues. 



Although the elastic reaction of the lung tissue during ex- 

 piration operates together with the elasticity of the thoracic 

 wall in diminishing the antero-posterior diameter of the 

 chest, it is chiefly effective in diminishing the vertical diam- 

 eter by raising the diaphragm. It exerts a certain "suction" 

 upon that muscle, causing it to arch upward in following the 

 contracting lungs. It is seen, therefore, that during inspira- 

 tion the chest wall and diaphragm exert "suction" upon the 

 lungs, causing them to follow, and during expiration the 

 lungs exert "suction" upon the chest wall and diaphragm, 

 causing them to follow. 



Forced Expiration. It is evident that, while ordinary ex- 

 piration is a passive process, a person can voluntarily force 

 out of his lungs more air than is ordinarily expelled, as in 

 singing, blowing, talking, etc. This is effected by certain 

 muscles whose contraction diminishes the thoracic capacity, 

 chiefly by depressing the ribs and elevating the diaphragm. 

 Those which depress the ribs are the intercostales internl, 

 infracostales and triangularis sterni. Those which elevate 

 the diaphragm do so by compressing the abdominal contents 

 and forcing them up against that muscle. They are the ob- 

 liquus externus, obliquus internus transver sails and rectus 

 abdominis. These depress the chest wall as well. 



Rhythm of Respiration. Under ordinary conditions in- 

 spiration and expiration follow each other in a regular rhyth- 

 mical fashion. Some hold that an interval follows inspira- 

 tion before expiration begins, but this is probably not cor- 

 rect. Indeed, it is doubtful if there be an interval following 

 expiration, though it will be here considered that there is a 

 brief one. Expiration is a little longer than inspiration. The 

 inspiratory act is of uniform intensity throughout, while the 

 expiratory act gradually diminishes in intensity as it ap- 

 proaches completion a circumstance to be expected from 

 the physical condition causing it. 



