168 OUR BODIES AND HOW WE LIVE 



The movements by which air is breathed in and out of 

 the lungs make up the act of respiration. 



227. How we breathe. The cavity of the chest is a 

 closed, air-tight chamber, whose only opening is the wind- 

 pipe. The pressure of the air in the air passages keeps 

 the lungs stretched out so as to fill this cavity. Imagine 

 now such a chamber as this to have a kind of floor, capable 



of moving up and down. When 

 the floor moves down, the cav- 

 ity will be enlarged, and the 

 pressure of the air inside the 

 lungs will then cause them to 

 expand to a greater extent to 

 fill up the extra space. 



When the floor is raised again, 

 the cavity will be diminished 

 and the lungs, being diminished 

 also, will give up the extra air 

 FIG. 105. BLACKBOARD SKETCH, which they have taken in. 



A Lobule of the Lung. The diaphragm serves as a 



A, an air sac ; B, an air sac cut open ; kind of movable floor to the 



C, capillary network over an air , ,-. 



sac; D, branch of pulmonary artery; Ctl ' rig. I 



, branch of pulmonary vein; F, These tWO movements are 



bronchial tube. r , 



performed at regular intervals 

 and constitute the process of breathing. 



228. Inspiration and Expiration. The cavity of the chest, 

 however, is enlarged in another way. The walls of the chest 

 are formed by the ribs, which encircle it and join the breast- 

 bone in front. The spaces between the ribs are filled with 

 a set of strong muscles called the intercostal muscles. 



One set of these intercostal muscles contracts, and pulls 

 up the ribs, which are fastened to the backbone behind by 



