Respiration 



169 



When the diaphragm contracts it descends and thus 

 increases the depth of the chest cavity. Being elastic, 

 the lungs expand, filling up the increased space, and a 

 quantity of air is drawn into them. As soon as the dia- 

 phragm relaxes, returning to its arched position and redu- 

 cing the size of the chest cavity, the 

 lungs diminish in size and the air is 

 driven out. After a short pause the 

 diaphragm again contracts, and the 

 same round of operations is repeated. 



By the contractions of the inter- 

 costals and other muscles the ribs 

 are raised and the breastbone pushed 

 forward. The chest cavity is thus 

 enlarged from side to side and from 

 behind forwards. Thus, by the simul- 

 taneous descent of the diaphragm and 

 the elevation of the ribs, the cavity 

 of the chest is enlarged in three direc- 

 tions, downwards, sideways, and 

 from behind forwards. 

 X It is thus evident that inspiration is 

 due to a series of muscular contractions. 



When the contractions cease the 

 walls of the chest resume their former 

 position and the diaphragm ascends. 

 As a result of these movements, the 

 original size of the chest cavity is 

 restored, and the inhaled air is driven from the lungs. 

 ^Expiration may then be regarded as the result of an 

 elastic recoil, and not of active muscular contractions. 

 / 257. The Nervous Control of Respiration. The move- 

 ments of breathing go on without our attention. In sleep 



FIG. 89. Diagrammatic 

 Section of the Trunk. 



(Showing the expansion of 

 the chest and the move- 

 ment of the ribs. The 

 dotted lines indicate the 

 position during inspira- 

 tion.) 



