154 



ELEMENTS OF PHYSIOLOGY 



But for the pleura there would be friction between the 

 lungs and the chest walls. 



257. The Diaphragm (Fig. 135). The floor of the 

 chest cavity is formed by a muscle that is the broadest in 

 the body, and also the thinnest in proportion to its width. 

 It is called the diaphragm. It rounds up under the concave 

 base of the lungs somewhat like a dome and separates the 

 thoracic and abdominal cavities. It is attached to the low- 

 est ribs at the sides and to the lumbar vertebrae behind 

 (Fig. 135). Its rounded side is turned toward the chest, and 

 its hollow side toward the abdomen. It is the most im- 

 portant muscle of the respiratory system. When it contracts, 

 it flattens and descends, and the lungs descend with it, thus 



lengthening and enlarging the 

 cavity of the chest from top to 

 bottom. 



When the diaphragm de- 

 scends, it acts as a piston or a 

 tight-fitting round board would 

 act if pressed down into a barrel 

 of water. If there were two 

 holes in the board (correspond- 

 ing to the vena cava and the 

 thoracic duct), the water would 

 be pressed up. Thus the circula- 

 FIG. 136. Figure showing Three tion is aided by breathing. When 



2M-Tia the dia P hra s ra relaxes > its thin - 



completing the Thoracic Wail. ness and flexibility would allow 



A, sternum; B, rib cartilage; C, verte- ft fa drOD downward, instead of 

 bral column; F, rib; G, outer inter- 

 costal muscles; H, inner intercostal Springing Upward as pupils SOmC- 



times suppose ; but the abdomi- 

 nal walls contract as the diaphragm relaxes and force the 

 liver, stomach, etc., against the diaphragm, thus pressing it 

 against the lungs (Figs. 137 and 138). 



258. Other Muscles that expand the Chest. The chest 



A 



G' 



