RESPIRATORY RHYTHM. 165 



is required. In all voluntary expiratory efforts, however, as 

 in speaking, singing, blowing, and the like, and in many in- 

 voluntary actions also, as sneezing, coughing, &c., 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 depress the ribs, 

 must act also as muscles of expiration, and therefore we must 

 conclude that the abdominal muscles are assisted in their ac- 

 tion by the greater part of the internal intercostals, the trian- 

 gularis sterui, the serratus posticus inferior, &c. When by 

 the efforts of the expiratory muscles, the chest has been 

 squeezed to less than its average diameter, it again, on relaxa- 

 tion of the muscles, returns to the normal dimensions by virtue 

 of its elasticity. The construction of the chest- walls, there- 

 fore, 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 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. 



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 inter- 

 vening 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 



