382 



HUMAN PHYSIOLOGY 



great increase in wall area by having a large number of tiny sacs 

 instead of one large one as in the frog. In the walls of the alveoli 

 are numerous capillaries, the ends of arteries which pass from 

 the heart into the lung. It is through the thin walls of the alveoli 

 that an interchange of gases takes place which results in the blood 

 giving up part of its load of carbon dioxide, and taking up oxygen 

 in its place. 



The Pleura. — The lungs are covered with a thin elastic mem- 

 brane, the pleura. This forms a bag in which the lungs are hung. 

 Between the walls of the bag and the lungs is a space filled with 

 lymph. By this means, the lungs are prevented from rubbing 

 against the walls of the chest. 



Breathing. — In every full breath there are two distinct move- 

 ments; inspiration (taking air in) and expiration (forcing air out). 

 Thus this action differs considerably from " breathing " movements 

 of the frog. The frog pumps air into its lungs by raising and lower- 

 ing the floor of the mouth and then shutting the flaps or valves in 

 the anterior nares. It is actually a process of swallowing. In man 

 the act of inspiration is partly under the control of the will. We 

 are able to take a long breath, or a short breath, though we can- 

 not stop breathing. An inspiration is produced by the contrac- 

 tion of the muscles between the ribs together with the contraction 



of the diaphragm, a muscular wall just below 

 the heart and lungs (not found in the frog); 

 this results in pulling down the diaphragm and 

 pulling upward and outward of the ribs, thus 

 making the space within the chest cavity 

 larger. The lungs, which lie within this cavity, 

 are filled by the air rushing into the larger 

 space thus made. An expiration is simpler 

 than an inspiration for it requires no muscular 

 effort; the muscles relax, the breastbone and 

 ribs sink into place, while the diaphragm re- 

 turns to its original position. 



Apparatus to illustrate 

 the action of the dia- 

 phragm in respira- 

 tion. 



A piece of apparatus which illustrates to a de- 

 gree the mechanics of breathing may be made as 

 follows : Attach a string to the middle of a piece of 

 sheet rubber. Tie the rubber over the large end of 



