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STUDIES IN PHYSIOLOGY 



abdominal walls must be stretched, when the stomach and 

 liver are forced downward. 



As soon as the muscles that cause these movements begin 

 to relax, the ribs sink back into their former position, the 

 breastbone is pulled back into place, and the distended wall 



FIG. 107. Diagram to show Changes in the Breastbone, Diaphragm, and 

 Abdominal Wall in Respiration. 



A = inspiration. D = diaphragm. 



B = expiration. St = breastbone or sternum. 



Ab = abdominal wall. Tr = windpipe. 



The shaded part is to indicate the stationary air. 



of the abdomen presses the organs upward against the dia- 

 phragm, which, therefore, becomes arched again. In all 

 these ways the walls of the chest cavity close in upon the 

 lungs, and thus help their elastic tissue to force out the air 

 in expiration. Ordinary expiration is thus accomplished 

 without muscular effort. 



