382 CIRCULATION AND RESPIRATION 



soda water into a tube. What we really do is to produce 

 a "partial vacuum" depending upon the air to make the 

 pressure in the lungs the same as that of the outside air. 



The expansion of the chest is brought about in two ways. One of 

 them is the contraction of the diaphragm, the muscular partition that 

 separates the thorax from the abdomen. When the diaphragm con- 

 tracts, it pushes the organs of the abdomen downward, and makes the 

 thorax larger. This permits the outer air to rush into the lungs. The 

 second means by which the expansion of the chest is brought about 

 is by the action of the chest muscles. When the diaphragm moves 

 downward, these muscles pull the breastbone and ribs outward. 



385. Expiration. The air taken into the lungs during 

 inspiration is expelled when the muscles of the chest and 

 diaphragm are relaxed. The ribs move downward and 

 inward of their own weight, and the organs of the abdo- 

 men, which were crowded together during inspiration, 

 expand again, and force the diaphragm up against the 

 lungs. As a result, the elastic air cells of the lungs, 

 which were il puffed out" with air, are now compressed, 

 and the air is forced back through the air passages. The 

 air that we inhale and exhale during ordinary quiet 

 breathing is called tidal air. It is about a pint (30 cubic 

 inches, or 500 c.c.) in volume. By making a conscious 

 effort we can take in and expel about 100 cubic inches 

 more air than in quiet breathing. The total capacity of 

 the lung is about 1.5 gallons (331 cubic inches; 5.7 liters). 

 An adult ' l breathes," on the average, 18 times in a minute. 



386. Exercises. 



1. Trace the course of a drop of blood from the time it leaves the 

 left ventricle until it re-enters it. 



