DIGESTION AND RESPIRATION 



531 



226. The Respiratory System of Terrestrial Vertebrates 



The lungs of early bony fishes evolved into hydrostatic swim blad- 

 ders in most of their descendants, but they were retained in some that 

 remained in fresh water, and it is from certain of these fishes that tetra- 

 pods evolved. Gills, which dry out on exposure to the air, have been 

 lost by adult amphibians, but are retained by their aquatic larvae. 

 Many larval amphibians, however, have external gills protruding from 

 the surface of the neck rather than ones within the gill slits. Adult 

 amphibians breathe by simple, saccular lungs, supplemented by a moist 

 skin and other mucous membranes. The somewhat awkward mouth 

 pump for moving air in and out of the lungs, and the need for aux- 

 iliary respiratory membranes, are among the factors that prevent am- 

 phibians from fully exploiting the terrestrial environment. The internal 

 surfaces of the lungs of higher letrapods have become greatly sub- 

 divided and have increased in area enough to dispense with respiration 

 in the skin. These organisms have also developed more efficient means 

 of ventilating the limgs. 



In mammals (Fig. 2(").7), air is drawn into the paired nasal cavities 

 through the external nares. These cavities are separated from the 

 mouth cavity by a bony palate, and the animal can breathe while food 

 is in its mouth. The surface area of the cavities is increased by a series 

 of ridges known as conchae, and the nasal mucosa (in addition to 

 having receptors for smell) is vascular, ciliated and contains many 

 mucous glands. In the nasal cavities the air is warmed, moistened and 



Concha 

 PaUte 



Epi^glottis 

 Tracheal rin^ 



Eusta.cKla.n tabg 



Nasopharynx 



Larynx. 

 Esopha-gus 



Pleura^, 



Bronchus 



Bronchiole 



Alveolar 

 duct and sa.cs 



Lung 

 (upper lobe) 



'erLcardiam. 



Lung 

 (lovv'erlobe) 



Diaphi-a.om. 



Figure 26.7. Respiratory system of man. Details of the alveolar sacs, here drawn 

 from above. A, normal position of vocal cords; B, position of cords during speech. 



