Chap. 13 



THE RELEASE OF ENERGY RESPIRATION 



233 



Nasal 

 capsule 



l^ood^^ <CC® 



Water 



A. FISH 



Food 



B. AMPHIBIAN 



Food 



REPTILE 



MAN 



Fig. 13.8. Diagrams of breathing and swallowing routes in aquatic and ter- 

 restrial vertebrates. Fishes inhale through the mouth. The routes of water and 

 food are parallel and entirely separate from the olfactory cavities. Beginning with 

 amphibians the routes of breathing and swallowing cross as they do in all other 

 air breathing vertebrates. The precise timing of nervous and muscular action 

 keeps the crossing clear for air or food. If both meet at the open trachea, choking 

 results. 



enlarge and contract the body cavity, drawing air in and out of the air sacs, 

 and through the lungs. During flight the pectoral muscles (white breast meat) 

 provide ventilation by moving the sternum (breastbone) toward and away 

 from the vertebral column. 



Tracheae of Insects. These airtubes extend throughout the body from open- 

 ings in the body wall and are the main distributors of oxygen (Chap. 30). 



Human Respiration 



Lungs. The human lungs begin as an outgrowth of the floor of the future 

 pharynx and develop a single trachea or windpipe which forks into two bron- 

 chial tubes (Fig. 13.10). Within each lung the bronchial tubes rebranch many 

 times and finally divide into minute bronchioles. Each bronchiole continues 

 into a small cluster of air sacs out of which minute alcoves or alveoli open 

 and create still further area for dift'usion of gases between air and blood 

 (Fig. 13.11). The bronchioles are encircled with smooth muscle innervated 



