320 



BBSPIKATION 



and the air is forced out of them by the passages above 

 noted. \Miat causes the lungs to expand and fill with 

 air, and, since the lungs themselves contain no muscles, 

 what parts of the body are concerned in this process? 

 The answer to this question is to be foimd in the structure 

 and movements of the walls of the chest cavity. 



Structure of the chest cavity. — The chest cavity is 

 conical in shape, with the point of the cone at the top, 

 and is completely inclosed on aU sides. The upper part 

 and sides of the cone are formed of the ribs, intercostal 

 muscles, sternum, and backbone, all covered over with 

 muscles and skin, while the floor of the cavity is com- 

 posed of the membrane called the diaphragm. These 

 walls are not rigid, but permit of movement in two defi- 

 nite directions. As we inhale, the sides of the cavity can 

 be felt to move outward, while the diaphragm is pulled 

 downward, forcing the contents of the abdominal cavity in 

 the same direction, and causing the belly to swell under 

 the pressure. As we exhale, the ribs move inward and the 

 diaphragm moves upward, causing the belly to be dra^\'n 

 in. The causes of these movements arp to be found by 

 studying the shape of the ribs and the muscular connec- 

 tions. 



In the dissection of the chest cavity, the muscles between 



Fig. 149 — Diagram of rib movement in breathing ; AB, vertebral oolnmn ; CD and 

 MF, ribs attached to movable breastbone, DF; a, 6, external and internal in- 

 tercostal muscles ; c, neck muscles. 



