56 HUMAN PHYSIOLOGY. 



Owing to the elastic tissue which is present in the lungs, they are very 

 readily distensible, so much so, indeed, that the pressure of the air inside 

 the trachea and lungs is sufficient to distend them until they completely 

 fill all parts of the thoracic cavity not occupied by the heart and great 

 vessels. The elastic tissue endows them not only with distensibility, but 

 also with the power of elastic recoil, by which they are enabled to accom- 

 modate themselves to all variations in the size of the thoracic cavity. 



When the chest walls recede, the air within the lungs expands and presses 

 them against the ribs ; when the chest walls contract, the air being driven 

 out, the elastic tissue recoils and the lungs return to their original condition. 

 The movements of the lungs are therefore entirely passive. 



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 evident 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 dimin- 

 ished 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 con- 

 dition 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 ten- 

 sion 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 presses 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 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. : 



