THE MECHANICS OF THE RESPIRATORY MOVEMENTS 471 



greatly hindered. A condition of this kind is commonly obtained dur- 

 ing pregnancy or when the thighs are flexed upon the abdomen. In the 

 nature of things, gravity cannot play a role in the upward movement 

 of the diaphragm unless the body be placed in a position directly 

 favoring this factor. The elastic recoil of the tissues, on the other 

 hand, is of paramount importance. In endeavoring to regain its 

 normal position, the stretched abdominal wall pushes the viscera and 

 overlying diaphragm upward. At this very moment the upper surface 

 of this now perfectly passive membrane is directly exposed to the elas- 

 tic recoil of the lungs. It will be seen, therefore, that the diaphragm 

 is made to assume its former position by two forces applied simul- 

 taneously to its under and upper surfaces. The elastic recoil of the 

 abdominal wall pushes it upward, while the elastic recoil of the lungs 

 pulls it upward. 



In forced expiration the capacity of the thorax is decreased still 

 further by the contraction of several abdominal and thoracic muscles. 

 Besides the internal intercostals and the triangularis sterni, mention 

 should be made at this time of the abdominales, serratus posticus 

 inferior, and quadratus lumborum. Obviously, the latter augment the 

 power of the recoil so that the abdominal organs are pushed against 

 the inferior surface of the diaphragm with an even greater force than 

 during normal expiration. 



Accessory Movements of Respiration. The muscles to which 

 reference has just been made, are concerned with the respiratory varia- 

 tions in the capacity of the thoracic cavity and hence, with the 

 expansion of the lungs. Besides these a number of other muscles 

 might be mentioned which give rise to certain associated respiratory 

 movements of the nostrils, pharynx and larynx. Thus, it may be 

 noticed that the orifices of the external nares are dilated during 

 inspiration and constricted during expiration. The former movement 

 is produced by the contraction of the elevators of the alse of the nose, 

 while the latter is the result of the elastic recoil of these parts. Ob- 

 viously, the purpose of this movement is to lessen the resistance to 

 the inflow of air. A similar rhythmic widening of the respiratory 

 passage occurs in the larynx, where the vocal cords are placed in 

 the path of the current of air for purposes of phonation. The space 

 between their free edges, the glottis, is enlarged during inspiration and 

 narrowed during expiration. The former effect is dependent upon 

 the contraction of the posterior crico-arytenoid muscles which abduct 

 the tips of the arytenoid cartilages to which the posterior extremities 

 of the vocal cords are attached. The muscles of the neck and face may 

 be made to contract rhythmically by rendering the respiratory move- 

 ments more labored (dyspnea). It is also claimed that the caliber 

 of the bronchi is increased during inspiration and decreased during 

 expiration. 



Classification of the Respiratory Muscles. The preceding dis- 

 cussion, no doubt, has shown that it is difficult to give a perfectly 



