RESPIRATION. 515 



During forced expiration certain muscles may be active, the chief being the 

 intercostales interni interossei, the triangulares sterni, the musculi abdominales, 

 and the levatores am. The intercostales interni interossei are probably active 

 expiratory muscles during forced expiration, but they can prove effective only 

 when the lower part of the thoracic cage is fixed or drawn down an act which 

 is accomplished chiefly by the abdominal muscles. 



The triangulares sterni pass outward and upward from the lower part of 

 the sternum, the inner surface of the ensiform cartilage, and the sternal 

 ends of the costal cartilages of the two or three lower sternal ribs, to the lower 

 and inner surfaces of the cartilages of the second to the sixth ribs, inclusive. 

 They draw the attached costal cartilages downward during expiration. 



The abdominales during quiet expiration are passive, and aid in the expul- 

 sion of air from the lungs simply by their elasticity ; but during forced expi- 

 ration, by contraction, they are active expiratory factors. 



The obliquus externus arises by slips on the outer surface and lower borders 

 of the lower eight ribs, and is inserted into the outer lip of the anterior half 

 of the crest of the ilium and into the broad aponeurosis which blends with 

 that of the opposite side in the linea alba. The obliquus internus passes 

 from the outer half or two-thirds of Poupart's ligament, the anterior two-thirds 

 of the middle lip of the crest of the ilium, and the posterior layer of the lumbar 

 fascia to the cartilages of the last three ribs and the aponeurosis of the anterior 

 part of the abdominal wall. The rectus abdominis passes from the crest of the 

 pubes and the ligaments in front of the symphysis pubis to the cartilages of 

 the fifth, sixth, and seventh ribs, and usually to the bone of the fifth rib. The 

 transversalis abdominis passes from the outer third of Poupart's ligament, the 

 anterior three- fourths of the inner lip of the iliac crest, by an aponeurosis 

 from the transverse and spinous processes of the lumbar vertebrae, and from 

 the inner surface of the sixth lower costal cartilages to the pubic crest and the 

 linea alba. The fibres for the most part have a horizontal direction. The pyram- 

 idalis passes from the anterior surface of the pubes and the pubic ligament 

 to the linea alba. It is obvious from the points of origin and insertion of the 

 abdominal muscles that during contraction they co-operate toward diminishing 

 the volume of the thorax in three ways : (1) By offering a base of action for 

 the internal intercostals, and thus aiding in the approximation of the ribs ; 

 (2) by depressing and drawing inward the lower end of the sternum and the 

 lower costal cartilages and ribs ; (3) by forcing the abdominal viscera against 

 the diaphragm, thrusting it upward. The abdominales are unquestionably 

 the chief expiratory muscles. 



The levatores am converge from the pelvic wall to the inner part of the rec- 

 tum and the prostate gland. They form the largest part of the muscular floor 

 of the pelvic cavity. The levatores ani are important' during forcible expi- 

 ration by resisting the downward pressure of the pelvic viscera caused by the 

 powerful contractions of the abdominal muscles, but they must be regarded rather 

 as associated in the act of expiration, and not as true expiratory muscles. 



Summary of the Actions of the Chief Muscles of Expiration. During 



