130 HUMAN PHYSIOLOGY. 



Respiratory Movements. The movements of respiration are two and 

 consist of an alternate expansion and recoil of the thorax, known as inspiration 

 and expiration. 



1. Inspiration is an active process, the result of the expansion of the thorax, 

 whereby the atmospheric air is introduced into the lungs. 



2. Expiration is a partially passive process, the result of the recoil of the 

 elastic walls of the thorax, and the recoil of the elastic tissue of the lungs 

 whereby the intrapulmonary air is expelled. 



In inspiration the chest is enlarged by an increase in all its diameters 

 viz.: 



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



2. The anteroposterior and transverse diameters are increased by the eleva- 

 tion and rotation of the ribs upon their axes. 



In ordinary tranquil 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 sternomastoid, 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 anteroposterior, by a depression of the ribs and sternum. 



In ordinary tranquil expiration the diameters of the thorax are diminished 

 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, intercostals, the infra- 

 costals, and the triangularis sterni. 



In the extraordinary efforts of expiration certain auxiliary muscles are 

 brought into play viz., the abdominal 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 contraction 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 respiratory 



