RESPIRATION. 



187 



chest and overcoming the elastic resistance of the lungs and chest-walls, 

 being returned as an expiratory effort when the muscles are relaxed. 

 This elastic recoil of the lungs is sufficient, in ordinary quiet breathing, 

 to expel air from the chest in the intervals of inspiration, and no muscular 

 power is required. In all voluntary expiratory efforts, however, as in speak- 

 ing, singing, blowing, and the like, and in many involuntary actions also, 

 as sneezing, coughing, etc., something more than merely passive elastic 

 power is necessary, and the proper expiratory muscles are brought into 

 action. By far the chief of these are the abdominal muscles, which, by 





FIG. 159. 



FIG. 160. 



FIG. 159. The changes of the thoracic and abdominal walls of the male during respiration. The 

 back is supposed to be fixed, in order to throw forward the respiratory movement as much as possi- 

 ble. The outer black continuous line in front represents the ordinary breathing movement: the ante- 

 rior margin of it being the boundary of inspiration, the posterior margin the limit of expiration. The 

 line is thicker over the abdomen, since the ordinary respiratory movement is chiefly abdominal ; thin 

 over the chest, for there is less movement over that region. The dotted line indicates the movement 

 on deep inspiration, during which the sternum advances while the abdomen recedes. 



FIG. 160. The respiratory movement in the female. The lines indicate the same changes as in 

 the last figure. The thickness of the continuous line over the sternum shows the larger extent of the 

 ordinary breathing movement over that region in the female than in the male. (John Hutcninson.) 



The posterior continuous line represents in both figures the limit of forced expiration. 



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, how- 

 ever, which depress 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 triangu- 

 laris sterni, the serratus posticus inferior, and quadratus lumborum. 

 When by the efforts of the expiratory muscles, the chest has been squeezed 

 to less than its average diameter, it again, on relaxation of the muscles, 

 returns to the normal dimensions by virtue of its elasticity. The con- 



