2io RESPIRATION. 



elasticity. The construction of the chest-walls, therefore, 

 admirably adapts them for recoiling against and resisting 

 as well undue contraction as undue dilatation. 



As before mentioned, the lungs, after distension in the 

 act of inspiration, contract by virtue of the elastic tissue 

 which is present in the bronchial tubes, on and between 

 the air-cells, and in the investing pleura. But in the 

 natural condition of the parts, they can never contract to 

 the utmost, but are always more or less on the stretch, 

 being kept closely in contact with the inner surface of the 

 walls of the chest by atmospheric pressure, and able to 

 contract away from these only when, by some means or 

 other, as by making an opening into the pleural cavity, or 

 by the effusion of fluid there, the pressure on the exterior 

 and interior of the lungs becomes equal. Thus, under 

 ordinary circumstances, the degree of contraction or dila- 

 tation of the lungs is dependent on that of the boundary 

 walls of the chest, the outer surface of the one being in 

 close contact with the inner surface of the other, and 

 obliged to follow it in all its movements. 



Respiratory Rhythm. 



The acts of expansion and contraction of the chest, take 

 up under ordinary circumstances a nearly equal time, and 

 can scarcely be said to be separated from each other by an 

 intervening pause. 



The act of inspiring air, however, especially in women 

 and children, is a little shorter than that of expelling it, 

 and there is commonly a very slight pause between the 

 end of expiration and the beginning of the next inspira- 

 tion. The respiratory rhythm may be thus expressed : 

 Inspiration ..... 6 



Expiration . . . . 7 or ^ 



A very slight pause. 



Respiratory Movements of the Glottis. 

 During the action of the muscles which directly draw 



