THE STEUCTURE AND FUNCTION OF THE ELEMENTARY LUNG 449 



In principle the structure of the gills is the same as that of the lungs. 

 In both cases the blood is brought into almost direct contact with the 

 medium, remaining separated from it only by a layer of flat endothe- 

 lial cells. Hence, the gills may be likened to a lung, which, so to 



Fig. 228. — Diagram Illustrating the Function of the Gills. 

 W, the water is driven across the surfaces of the gill-plates, whence O diffuses into the 

 gill capillaries and CO2 out of them. 



speak, has been turned inside out (Fig. 228). Naturally, the size of 

 the respiratory surface of this organ differs greatly in accordance with 

 the metabolism of the different animals. The individual plates 

 become more numerous and frequently extend as fringed folds far out 



Fig. 229. — Diagram Illustrating the Flow of the Water Through the Mouth 

 Cavity of A Bony Fish. {After Dahlgren.) 

 A, inspiration; B, expiration; M, cavity of the mouth; D, esophagus; G, gills; 

 MV, maxillary valve; BV, bronchostegal valve; OP, operculum moves outward on 

 inspiration, opening MV and closing BV. On expiration operculum moves inward, 

 closing first valve and opening second valve. 



into the water. These gill-plates are supplied with venous blood 

 which after its oxygenation is returned into the dorsal aorta. 



In illustration of the method by means of which the individual 

 plates are constantly supplied with fresh water, we might briefly 

 consider the respiratory mechanism in the teleosts (Fig. 229). An 



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