324 BESPIRA.TIOK 



inhaling and exhaling, and this process may be repeated 

 as often as we lower and raise the rubber diaphragm. 



In the chest cavity the conditions are practically iden- 

 tical with that of our apparatus, but the hmgs are much 

 more sensitive to changes in pressure than are the balloon 

 walls. One reason for this is that the chest cavity volume 

 is increased not only by the downward movement of the 

 diaphragm but also by the upward movements of the ribs. 

 Furthermore, the lungs fit closely against the walls of the 

 cavity, and thus a slight increase in volume is immediately 

 felt by all parts of the lungs, with a correspondingly greater 

 expansion. 



The movements of contraction and expansion, on the 

 part of the chest walls, take place rhythmically about fifteen 

 to eighteen times per minute, which is the normal rate 

 of breathing. In violent exercise this rate may increase 

 to sixty or seventy times per minute. The rate also varies 

 with the age, that for a baby being about forty, while this 

 rate gradually decreases, until, at eighteen years of age, it 

 is practically the same as for an adult. Much more mus- 

 cular effort is exerted in inhaling than in exhaling, there 

 being practically none in normal exhaling. 



Physiological Effects of Breathing. 



Composition of inspired and expired air. — The study of 

 ordinary air (see p. 20) shows it to be composed mainly 

 of two gases, oxygen and nitrogen. Careful chemical 

 analysis shows the presence of small quantities of other 

 chemical elements and compounds, such as argon, krypton, 

 carbon dioxide, and water. Of these, only the water and 

 carbon dioxide have any physiological significance. Ex- 



