BREATHING IN MAN 



149 



movements. The ventilation of the lungs 

 is brought about by the action of muscles 

 in organs outside the lungs. There are 

 two sets of organs that are concerned in 

 these movements : the ribs, with their 

 connected muscles, and the diaphragm 

 (see Fig. 48). 



When the muscles of the ribs and of 

 the diaphragm relax, the chest cavity 

 shrinks, and this forces the lungs to 

 collapse, driving the air out. By the 

 alternate expansion and contraction of 

 the chest, inspiration and expiration, the 

 two movements of air in respiration, are 

 brought about. 



The external respiration of the body 

 consists of (i) the muscular movements 

 of the ribs and the diaphragm; (2) the 

 air movements into and out of the lungs ; 

 and (3) the osmotic movements of gases 

 into and out of the blood, through the 

 lining of the air cells. 



The internal respiration of the body 

 cells consists of the gas exchange be- 

 tween the cells and the blood or lymph. 

 Internal and external respiration are re- 

 lated to the vital process of energesis. 



180. Control of breathing. When you 

 wish to do so, you may hold your breath 

 for a minute or two ; you may breathe 

 faster or slower; you may breathe with 

 your diaphragm, holding your chest wall 

 nearly rigid, or you may breathe with your chest, keeping the 

 abdomen almost immovable. Nevertheless the process of 

 breathing, as it is carried on hour by hour and day by day, 



FIG. 48. The movements 

 of breathing in man 



When the muscular parti- 

 tion (called the diaphragm) 

 between the chest cavity 

 and the abdominal cavity 

 is pulled down, the chest 

 cavity is enlarged. When 

 the ribs are raised, the chest 

 cavity is also enlarged. The 

 rib muscles and the dia- 

 phragm normally work in 

 unison, alternately expand- 

 ing and contracting the 

 chest cavity. The shaded 

 portion of the diagram 

 shows the expanded con- 

 dition ribs raised and 

 diaphragm lowered 



