RESPIRATION. 57 



1. The vertical is increased by the contraction and descent of the dia- 

 phragm when it approximates a straight line. 



2. The antero-posterior and transverse diameters are increased by the 

 elevation and rotation of the ribs upon their axes. 



In ordinary tranqziil inspiration the muscles which elevate the ribs 

 and thrust the sternum forward, and so increase the diameters of the chest, 

 are the external intercostals, running from above downward 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 sterno-mastoid, pectorales, serratus magnus, 

 which increase the capacity 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 antero-posterior, by a depression of the ribs and sternum. 



In ordinary tranquil expiration the diameters of the thorax are dimin- 

 ished by the recoil of the elastic 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 inter- 

 costals, the infracostals, and the triangularis sterni. 



In the extraordinary efforts of expiration certain auxiliary muscles are 

 brought into play, viz. : the abdominal and sacro-lumbalis 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 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 contraction of 

 the crico-arytenoid muscles, so as to freely admit the passage of air into the 

 lungs : in expiration they fall passively together, but do not interfere with 

 the exit of the air from the chest. 



Nervous Mechanism of Respiration. The movements of Respi- 

 ratory muscles, though capable of being modified to a certain extent by 

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

 nervous impulses emanating from the medulla oblongata. The Respiratory 

 centre, the so-called vital point, generates the nerve impulses, which, trav- 

 eling outward through the phrenic and intercostal nerves, excite contrac- 

 tions of the diaphragm and intercostal muscles respectively. This centre 

 is for the most part automatic in its action, though it is capable of being 

 modified by impulses reflected to it .through various sensory nerves. 



This centre may be stimulated 

 E 



