Chap. 13 



THE RELEASE OF ENERGY — RESPIRATION 



237 



Internol Nares 

 Uvula 



Nosol Cavity 



External Nores 



Fig. 13.12. Human respiratory system. (Courtesy, Villee: Biology. The H union 

 Approach, ed. 2. Philadelphia, W. B. Saunders Co., 1954.) 



Mechanism of Breathing 



Inspiration. Lungs expand because the chest cavity pulls upon them from 

 all sides. Nerve impulses from the respiratory center in the medulla stimulate 

 the intercostal muscles to contract, which means lifting the ribs (Fig. 13.13). 

 The ribs then move outward as they are lifted, like the handle of a pail, and 

 thus they increase the spread of the chest. The breastbone also moves up carry- 

 ing the front ends of the ribs with it, and increasing the chest cavity from front 

 to back. At the same time impulses from the respiratory center are relayed 

 over the phrenic nerves to the diaphragm and make it contract, deepening the 

 chest cavity. Instead of a low dome pressing up into the thoracic cavity, the 



