RESPIRATION. 



145 



and forward ; the sternal portion of the internal intercostals, and the 

 levatores costarum. 



In the extraordinary efforts of inspiration certain auxiliary muscles 

 are brought into play, viz., the sternomastoid, pectorales, serratus 

 magnus, which increase the capac- 

 ity of the thorax to its utmost limit. 



In expiration the diameters of the 

 chest are all diminished viz. : 



1. The vertical, by the ascent of the 

 diaphragm. 



2. The anteroposterior, by a depres- 

 sion of the ribs and sternum. 



In ordinary tranquil expiration 

 the diameters of the thorax are 

 diminished by the recoil of the elas- 

 tic tissue of the lungs and the 

 ribs ; but in forcible expiration the 

 muscles which depress the ribs and 

 sternum, and thus further diminish 

 the diameter of the chest, are the 

 internal, intercostals, the infracos- 

 tals, and the triangularis sterni. 



In the extraordinary efforts of ex- 

 piration certain auxiliary muscles 

 are brought into play, viz., the ab- 

 do.minal and sacrolumbalis muscles, 

 which diminish the capacity of 

 the thorax to its utmost limit. 



Expiration is aided by the recoil of the elastic tissue of the 

 lungs and ribs and by the pressure of the air. 



Movements of the Glottis. At each inspiration the rima glottidis 

 is dilated by a separation of the vocal cords, produced by the con- 

 traction of the crico-arytenoid muscles, so as freely to admit the 

 passage of air into the lungs ; in expiration they fall passively 

 together, but do not interfere with the exit of air from the chest. 



Nerve Mechanism of Respiration. The movements of respira- 

 tory muscles, though capable of being modified to a certain extent 

 by efforts of the will, are of an automatic character, and called 

 forth by nerve impulses emanating from the medulla oblongata. 

 11 



FIG. 1 8. DIAGRAM OF THE RESPI- 

 RATORY ORGANS. 



The windpipe, leading down from 

 the larynx, is seen to branch 

 into two large bronchi, which 

 subdivide after they enter 

 their respective lungs. 



