ELASTIC RECOIL OF LUNGS AND CHEST. 199 



actions also, as sneezing, coughing, etc., something more 

 than merely passive elastic power is of course necessary, 

 and the proper expiratory muscles are brought into action. 

 By far the chief of these are the abdominal muscles, which, 

 by pressing on the viscera of the abdomen, push up the 

 floor of the chest formed by the diaphragm, and by thus 

 making pressure on the lungs, expel air from them through 

 the trachea and larynx. All muscles, however, which de- 

 press the ribs, must act also as muscles of expiration, and 

 therefore we must conclude that the abdominal muscles are 

 assisted in their action by the greater part of the internal 

 intercostals, the triangularis sterni, the serratus posticus 

 inferior, etc. WheifBy the efforts of the expiratory muscles, 

 the chest has been squeezed to less than its average dia- 

 meter, it again, on relaxation of the muscles, returns to 

 the normal dimensions by virtue of its elasticity. The 

 construction of the chest-walls, therefore, admirably adapts 

 them for recoiling against and resisting as well undue con- 

 traction 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, able to act only 

 on their interior, and can 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 dilatation 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. 



