RESPIRATION. 353 



The action of the band is supposed to disclose and illustrate the 

 action of the muscle. 



The serratus posticus injerior arises from the spines of the last 

 two thoracic and first two lumbar vertebras. The fibers pass upward 

 to be inserted into the lower border of the last four or five ribs beyond 

 the angle. Their action is to depress the ribs and assist in expira- 

 tion. 



The quadratus lumborum has a similar action on the last rib. 



Movements of the Lungs. As the thorax is enlarging in all its 

 diameters during inspiration, through muscle activity, the lungs are 

 correspondingly enlarging in all their diameters, by virtue of their 

 distensibility, through the pressure of the air within them. The 

 lungs must therefore move downward, outward, and forward. That 

 this is the case is made evident both by an examination of the lungs 

 through an intercostal space after removal of the skin and intercostal 

 muscles and by the methods of percussion. The inferior border of 

 each lung descends from the lower border of the sixth to the eleventh 

 rib, inserting itself into the space developed between the thorax and 

 diaphragm as the latter contracts and is drawn away from the former. 

 In consequence of the lateral expansion the anterior border of each 

 lung advances toward the middle line until the heart is almost cov- 

 ered. With the beginning and continuance of expiration the lungs 

 exhibit a reverse movement which continues until they reach their 

 original position.^At all times, however, the movements of the lungs 

 are entirely passive and determined by the movements of the thorax.W 



Respiratory Movements of the Upper Air-passages. The X 

 resistance to the entrance of air into and through the respiratory 

 tract is much diminished by respiratory movements of the nares and 

 larynx which are associated and occur synchronously with the move- 

 ment of the thorax. 



The nares at each inspiration are dilated by the outward move- 

 ment of their alre or wings, the result of muscle activity. At each 

 expiration they are diminished by the return of their cartilages through 

 the play of elastic forces. The larynx, as shown by observation 

 with the laryngoscope, exhibits corresponding movements of the vocal 

 membranes. Their introduction at this level naturally narrows the 

 tract, and would interfere with both the entrance and the exit of air 

 were they not kept widely asunder during the time they are not re- 

 quired for purposes of phonation.^ This is accomplished by the 

 tonic contraction of the posterior crico-arytenoid muscles, which are 

 entirely respiratory in function^ 



It is not infrequently stated that these membranes exhibit consider- 

 able oscillations, outward and inward, corresponding to the periods 

 of inspiration and expiration. The statements of the majority of 

 laryngologists do not favor this view. During tranquil breathing 

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