l62 



BIOLOGY AND HUMAN LIFE 



its oxygen from its immediate neighbor- 

 hood, and (2) discharges its carbon 

 dioxid and other products of oxidation 

 into its immediate surroundings. 



136. Breathing in man. Breathing, or 

 respiration, means a process of gas ex- 

 change — taking in oxygen and giving off 

 carbon dioxid. It makes oxidation pos- 

 sible. In man (as, in fact, in all back- 

 boned animals except fishes and the 

 young of amphibians) air is taken from 

 the outside into the lungs (soft bags sus- 

 pended in the thorax, or chest cavity), 

 and carbon dioxid is discharged to the 

 exterior from the same organs (see 

 Fig. 83). The lungs consist of air sacs 

 (which are lined with a layer of thin- 

 walled cells and surrounded by very fine 

 blood vessels) and air tubes (see Fig. 83 ) . 

 The filling of the air sacs with fresh 

 air and the emptying of the lungs are 

 brought about by the action of (i) mus- 

 cles attached to the ribs and (2) a large 

 muscular organ called the diaphragm 

 (di'a fram). This separates the chest 

 cavity from the abdominal cavity 

 (Fig. 84). When the muscles of the ribs 

 and of the diaphragm relax, the chest 

 cavity shrinks ; this forces the air out of 

 the lungs. By contracting the muscles 

 of the ribs and the diaphragm we force 

 the chest cavity to enlarge, reducing the 

 pressure in the lungs and drawing air 

 into them. Inspiration and expiration, the two movements of 

 air in respiration, are thus brought about by the alternate ex- 

 pansion and contraction of the thoracic cavity. There are sev- 



Fig. 84.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 



