62 HUMAN PHYSIOLOGY. 



a manometer into the trachea. Upon opening the thorax and allowing the 

 tissue to recoil, the air presses upon the mercury and elevates it, the extent 

 to which it is raised being the index of the pressure. Hutchinson calcu- 

 lated the pressure to be one-half pound to the square inch of the lung 

 surface. 



Respiratory movements. The movements of respiration are two, and 

 consist of an alternate dilatation and contraction of the chest, known as 

 inspiration and expiration. 



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

 thorax, whereby 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 carbonic acid 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 dia- 

 phragm when it approximates a straight line. 



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

 elvation 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 inter costals, running from above downward and forward, the 

 sternal portion of the internal intercostah 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 infracostal*, and the triangularis sterni. 



In the extraordinary efforts of expiration certain auxiliary muscles are 

 brought into plaj, 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. 



