RESPIRATION 125 



ninth, tenth, eleventh, and twelfth ribs. The scaleni fix the 

 first and second ribs. The serrati postici superiores help to 

 fix the second rib, and raise the third, fourth, and fifth ribs. 

 The external intercostals and interchondrals and the levatores 

 costarum elevate and evert the first to the tenth ribs. They 

 serve also to give the intercostal tissue a proper tension. 



During forced inspiration additional muscles are brought 

 into play to permit a more powerful inspiratory act. Besides 

 the muscles already enumerated, the following are brought 

 into play: The trapezei and rhomboidei fix the shoulders; 

 the pectorales majores and minores, acting from the fixed 

 shoulders, draw the sternum and ribs upward; the sterno- 

 mastoidea fix the upper part of the chest; the erectores spinse 

 stiffen the vertebral column; the serrati postici inferiores, quad- 

 rati lumborum, and sacrolumbales draw the lower ribs down- 

 ward and backward. 



At the close of inspiration the various muscles that raised 

 the thorax gradually relax, and by its weight the thorax com- 

 presses the lungs and expels the air. In addition there is an 

 active recoil of the elastic tissue in the substance of the lung, 

 which has been put on the stretch during inspiration. Also 

 during inspiration the interosseous portions of the internal 

 intercostal muscles were put on a stretch; when expiration 

 begins .these muscles contract, but their contraction is not 

 sufficiently forcible to pull the ribs down, and the only purpose 

 of this contraction seems to be to keep the intercostal tissues 

 tense and thus prevent bulging of the intercostal spaces. Dur- 

 ing inspiration each costal cartilage is twisted in the direction 

 of its long axis by the e version of the ribs. During expiration 

 the costal cartilage tends to untwist itself. It may be said 

 there are no muscles of quiet expiration. Forced expiration 

 is accomplished by the intervention of many muscles. 



The interosseous internal intercostals act forcibly in draw- 

 ing down the ribs when the lower part of the thorax is fixed; 

 the abdominal muscles fix the lower part of the thorax and 

 press the abdominal contents upward; the levatores ani and 

 perineal muscles hold the floor of the pelvis rigid against abdomi- 

 nal pressure; the triangularis sterni draws the costal cartilages 

 down. 



A number of movements which take place in connection 



