144 HUMAN PHYSIOLOGY. 



As the capacity of the chest in a state of rest is greater than 

 the volume of the lungs after they are collapsed, it is quite evi- 

 dent that in the living condition the lungs are distended and in a 

 state of elastic tension, which is greater or less in proportion as 

 the thoracic cavity is increased or diminished in size. The elastic 

 tissue, always on the stretch, is endeavoring to pull the visceral 

 layer of the pleura away from the parietal layer, but is antagonized 

 by the pressure of the air within the air-passages. This condition 

 of things persists as long as the thoracic cavity remains air-tight ; 

 but if an opening be made in the thoracic wall, the pressure of the 

 external air, which was previously supported by the practically rigid 

 walls of the thorax, now presses upon the lung with as much force 

 as the air within the lung. The two pressures being neutralized, 

 there is nothing to prevent the elastic tissue from recoiling, driving 

 the air out, and collapsing.- The elastic tension of the lungs can be 

 readily measured in man after death by inserting a manometer into 

 the trachea. Upon opening the thorax and allowing the tissue to 

 recoil, the air passes upon the mercury and elevates it, the extent 

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

 calculated the pressure to be one half pound to the square inch of 

 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 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 elevation 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 



