144 RESPIRATION 



chiefly effective in diminishing the vertical diameter 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 inspiration the chest wall and 

 diaphragm exert "suction" upon the lungs, causing them to fol- 

 low, 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 expi- 

 ration is a passive process, a person can voluntarily force out of 

 his lungs more air than is ordinarily expelled, as in singing, blow- 

 ing, talking, etc. This is effected by certain muscles whose con- 

 traction diminishes the thoracic capacity, chiefly by depressing 

 the ribs and elevating the diaphragm. Those which depress the 

 ribs are the intercostales interni, 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 obliquus externus, obliquus internus transversalis 

 and rectus abdominis. These depress the chest wall as well. 



Rhythm of Respiration. Under ordinary conditions inspi- 

 ration and expiration follow each other in a regular rhythmical 

 fashion. Some hold that an interval follows inspiration before 

 expiration begins, but this is probably not correct. 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 uni- 

 form intensity throughout, while the expiratory act gradually 

 diminishes in intensity as it approaches completion a circum- 

 stance to be expected from the physical conditions causing it. 



After every six to ten respiratory acts a more profound (sigh- 

 ing) inspiration than usual is taken, the effect being a more 

 thorough changing of the pulmonary contents. Coughing, 

 sneezing, hiccoughing, laughing, etc., all interfere with rhythmical 

 respiration. 



Modified Respiration. In coughing and sneezing a profound 



