THE RESPIRATORY SYSTEM 



93 



air in the bell-jar through the tube with the pinch-cock attachment 

 a negative pressure may be made. If the sheet of rubber at the 

 bottom of the apparatus is pulled downward, the size of the cavity 

 will be increased and the pressure still further lowered, causing the 

 air to rush in through the trachea and expand the lungs (L). 

 When the pull is released, the pleural pressure rises, and the lungs 

 tend to collapse and force the air out. The bell-jar used as a model 

 for our illustration was larger than necessary, and may give a wrong 



Fig. 27. — Apparatus to illustrate the mechanics of respiration: L, Lungs 

 of experimental animal; D, rubber diaphragm. At the left the lungs are col- 

 lapsed and the diaphragm relaxed, as occurs after expiration; at the right the 

 diaphragm is contracted, which, increases the negative pressure around the 

 lungs and allows the air to rush into and expand them. 



impression, unless it is remembered that in life the lungs and other 

 organs completely fill the thoracic cavity, so that no air space exists 

 between them. 



Respiration is the act of breathing. Obviously, it is the most 

 vital function of animals. It consists of (A) an exchange of the 

 oxygen in the air for the carbon dioxid in the blood, and (B) an 

 interchange of these gases between the blood and the body tissues. 

 The first exchange is termed "external respiration"; the second, 

 "internal respiration." 



