488 VERTEBRATE LIFE AND ORGANIZATION 



Diaphragm 



(— Plcu.ra.1 cavity 

 "Lung 



Esophagus 

 I^harynx 



Eustachi an tobc 



Brain. 



-N/^IntarnsLl naris 



Nasal 

 cavity 



External 

 naris 



— Mouth cavity 

 Secondary palate. 



Larynx 

 "-Trachea 

 ■Heart 

 -Alveoli (greatly enlcirged) 



Figure 24.13. A sagittal section of the head of a pig showing the relationship be- 

 tween the digestive and respiratory systems. The route of air is shown by arrows. 



improved methods of ventilation (Fig. 24.13). The increase in surface is 

 accomplished by a subdivision of the air passages within the lung so 

 that all end in clusters of thin-walled sacs (alveoli) whose walls contain 

 a dense capillary network. It has been estimated that the respiratory 

 surface of the human lungs is between 50 and 100 square meters, or 25 

 to 50 times the surface area of the body. Birds also have a large respira- 

 tory surface, but their lungs are more compact organs and the respiratory 

 surface may not be relatively as great as in mammals. Birds and mam- 

 mals differ in the method of ventilation. Air must be moved in and out 

 of blind sacs in mammalian lungs, whereas there can be a through draft 

 in avian lungs. The lungs of birds are more efficient as gas exchangers, 

 for the air in the air capillaries contains relatively more oxygen than 

 the somewhat stale air in the alveoli, but the more thorough ventilation 

 of avian lungs probably results in a greater loss of body water via this 

 route. 



The mechanics for the ventilation of mammalian lungs are more 

 efficient than those of amphibians and reptiles. One important factor in 

 improved ventilation has been the evolution of a muscular diaphragm 

 whose contractions, together with a forward movement of the ribs, 

 expand the chest cavity and draw air into the lungs. Another factor has 

 been the evolution of a secondary palate, a horizontal partition of bone 

 and flesh in the roof of the mouth that separates the air and food pas- 

 sages in this region. In lower tetrapods, the nasal cavities lead directly 

 into the front of the mouth, but in mammals they open more posteriorly 

 into the pharynx. The secondary palate permits nearly continuous 



