194 RESPIRATION, BLOOD, AND EXCRETION 



apple." The larynx is formed by several large pieces of 

 cartilage lined with a mucous membrane. On the inside 



ical cords 

 trachea 



During Respiration During Phonafion 



Figure 207. — Voice Box or Larynx. 



of the larynx project two folds of elastic tissue which are 

 called the vocal cords. 



157. Breathing. — The lungs are elastic and can be 

 squeezed like a sponge. Inspiration is the term applied 

 to the taking of air into the lungs, and expiration to the 

 forcing out of air. When air is drawn into the lungs, 

 the chest expands, and the diaphragm (Figure 208), the 

 horizontal muscle which divides the lung cavity from the 

 abdomen, is drawn down. Thus the chest cavity is en- 

 larged and air is sucked into the lungs. In expiration 

 the air passes out gently. 



When we breathe naturally, only a small part of the 

 air in the lungs is exchanged at each inspiration and ex- 

 piration, but by breathing deeply a few times we can 

 remove the larger part of the air from the lungs and re- 

 place it with fresh air. 



The natural rate of breathing is about eighteen times a 

 minute, but the rate is higher in persons with a small lung 

 capacity. Exercise increases the rate of breathing. Ex- 



